Health Education Systems Inc (HESI) A2 Exam Quiz: Acids Bases And Ph
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Acids Bases And PhQuestion 1 of 20

In the Brønsted-Lowry theory, a conjugate acid is formed when a base accepts a proton. What is the conjugate acid of the bicarbonate ion, HCO3−\text{HCO}_3^-?

Carbonate ion (CO32−\text{CO}_3^{2-})
Hydroxide ion (OH−\text{OH}^-)
Carbonic acid (H2CO3\text{H}_2\text{CO}_3)
Hydrogen ion (H+\text{H}^+)
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Health Education Systems Inc (HESI) A2 Exam Quiz

Health Education Systems Inc (HESI) A2 Exam Quiz: Acids Bases And Ph

Practice Acids Bases And Ph in Health Education Systems Inc (HESI) A2 Exam with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Acids Bases And Ph, giving you a quick way to practice the rules, question types, and explanations that matter most for Health Education Systems Inc (HESI) A2 Exam.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

In the Brønsted-Lowry theory, a conjugate acid is formed when a base accepts a proton. What is the conjugate acid of the bicarbonate ion, HCO3−\text{HCO}_3^-?

  1. Carbonate ion (CO32−\text{CO}_3^{2-})
  2. Hydroxide ion (OH−\text{OH}^-)
  3. Carbonic acid (H2CO3\text{H}_2\text{CO}_3) (correct answer)
  4. Hydrogen ion (H+\text{H}^+)
Explanation: Understanding acid-base chemistry requires mastering the Brønsted-Lowry theory, which defines acids as proton donors and bases as proton acceptors. When dealing with conjugate acid-base pairs, remember that a conjugate acid forms when a base gains a proton (H⁺), while a conjugate base forms when an acid loses a proton. To find the conjugate acid of bicarbonate ion (HCO3−\text{HCO}_3^-), you need to determine what happens when this base accepts a proton. Starting with HCO3−\text{HCO}_3^- and adding H⁺ gives you H2CO3\text{H}_2\text{CO}_3 (carbonic acid). The reaction is: HCO3−+H+→H2CO3\text{HCO}_3^- + \text{H}^+ \rightarrow \text{H}_2\text{CO}_3 Option C is correct because carbonic acid is exactly what forms when bicarbonate accepts a proton. Option A (carbonate ion, CO32−\text{CO}_3^{2-}) is incorrect because this would be the conjugate base of bicarbonate—what you'd get if bicarbonate lost a proton rather than gained one. Option B (hydroxide ion, OH−\text{OH}^-) is completely unrelated to the bicarbonate system and represents a different base entirely. Option D (hydrogen ion, H+\text{H}^+) is the proton itself, not a conjugate acid formed by accepting a proton. Remember this pattern: to find a conjugate acid, add H⁺ to the original species; to find a conjugate base, remove H⁺. Also, notice how the charge changes—bicarbonate's -1 charge becomes neutral (0) in carbonic acid after gaining the +1 proton. This charge accounting helps verify your answer.

Question 2

A laboratory technician prepares two acidic solutions. Solution A has a pH of 3, and Solution B has a pH of 5. How does the hydrogen ion concentration, [H+], of Solution A compare to that of Solution B?

  1. Solution A has 2 times the [H+] of Solution B.
  2. Solution A has 100 times the [H+] of Solution B. (correct answer)
  3. Solution B has 100 times the [H+] of Solution A.
  4. Solution A has 20 times the [H+] of Solution B.
Explanation: When you encounter pH problems, remember that pH is a logarithmic scale measuring hydrogen ion concentration. The key relationship is: pH=−log⁡[H+]pH = -\log[H^+], which means [H+]=10−pH[H^+] = 10^{-pH}. To compare these solutions, calculate each hydrogen ion concentration. Solution A (pH = 3): [H+]=10−3=0.001 M[H^+] = 10^{-3} = 0.001 \text{ M}. Solution B (pH = 5): [H+]=10−5=0.00001 M[H^+] = 10^{-5} = 0.00001 \text{ M}. To find how many times greater Solution A's concentration is, divide: 10−310−5=10−3−(−5)=102=100\frac{10^{-3}}{10^{-5}} = 10^{-3-(-5)} = 10^2 = 100. Solution A has 100 times the hydrogen ion concentration of Solution B. Looking at the wrong answers: Choice A suggests only 2 times the difference, which ignores the logarithmic nature of pH and incorrectly treats it as a linear scale. Choice C reverses the relationship entirely—it claims Solution B (higher pH, lower acidity) has more hydrogen ions than Solution A, which contradicts basic acid-base principles. Choice D gives 20 times, which might result from incorrectly multiplying the pH difference (5-3=2) by 10 instead of using proper exponential calculations. Study tip: Remember that each unit decrease in pH represents a 10-fold increase in hydrogen ion concentration. When comparing pH values that differ by 2 units, the concentration difference is always 102=10010^2 = 100 times. This logarithmic relationship is fundamental to acid-base chemistry questions on the HESI.

Question 3

A student mixes a solution of hydrochloric acid (HCl), a strong acid, with a solution of potassium hydroxide (KOH), a strong base. Assuming the reactants are in stoichiometrically equivalent amounts, what are the primary products of this reaction?

  1. Potassium chloride (KCl) and water (H2O) (correct answer)
  2. Potassium hydride (KH) and chloric acid (HClO3)
  3. Hydrogen gas (H2) and potassium hypochlorite (KClO)
  4. Only water (H2O), as the acid and base are fully neutralized
Explanation: When you encounter acid-base reactions on the HESI, focus on the fundamental principle that acids and bases neutralize each other to form salt and water. This is one of the most predictable reaction types in chemistry. In this reaction, hydrochloric acid (HCl) donates a hydrogen ion (H⁺), while potassium hydroxide (KOH) donates a hydroxide ion (OH⁻). The complete reaction is: HCl+KOH→KCl+H2O\text{HCl} + \text{KOH} \rightarrow \text{KCl} + \text{H}_2\text{O} The H⁺ from the acid combines with the OH⁻ from the base to form water (H₂O), while the remaining ions (K⁺ and Cl⁻) form the salt potassium chloride (KCl). Since both reactants are strong and present in stoichiometrically equivalent amounts, the reaction goes to completion, producing these two products. Answer A is correct because it identifies both products of this classic acid-base neutralization. Answer B is wrong because potassium hydride and chloric acid aren't formed in simple neutralization reactions—these would require entirely different reaction conditions. Answer C incorrectly suggests hydrogen gas production, which occurs in metal-acid reactions, not acid-base neutralizations, and potassium hypochlorite isn't formed here. Answer D is incomplete because while water does form, it ignores the salt product that always accompanies neutralization reactions. Remember this pattern: strong acid + strong base = salt + water. The salt is always formed from the metal cation (K⁺) and the acid's anion (Cl⁻). This formula works for virtually all acid-base neutralization problems you'll see on the HESI.

Question 4

If a beaker of pure water at pH 7 is used to dilute a solution of nitric acid (HNO3) with a pH of 1, what will be the resulting effect on the pH of the nitric acid solution?

  1. The pH will decrease, moving towards 0.
  2. The pH will increase, moving towards 7. (correct answer)
  3. The pH will remain at 1 because nitric acid is a strong acid.
  4. The pH will become greater than 7 because water is being added.
Explanation: When you encounter pH and dilution problems, remember that pH measures hydrogen ion concentration on a logarithmic scale. Adding water to an acidic solution always dilutes the acid, reducing the concentration of hydrogen ions and raising the pH toward neutral. Here's what happens when you dilute the nitric acid solution: The original solution has a pH of 1, meaning it contains a high concentration of H⁺ ions. When you add pure water (pH 7), you're increasing the total volume while keeping the same amount of acid. This spreads the H⁺ ions over a larger volume, reducing their concentration. Since pH is inversely related to H⁺ concentration, the pH increases, moving closer to 7. Looking at the wrong answers: Choice A suggests the pH decreases toward 0, which would mean the solution becomes more acidic—this contradicts the basic principle that dilution reduces concentration. Choice C incorrectly assumes that being a strong acid means the pH won't change upon dilution. While HNO₃ is indeed a strong acid (fully ionizes), dilution still affects the concentration of those ions. Choice D suggests the pH will exceed 7, making the solution basic. However, adding neutral water to an acidic solution can only move the pH toward neutral, never past it to the basic side. For HESI chemistry questions, remember this key principle: dilution always moves pH toward neutral (7). Acidic solutions become less acidic, and basic solutions become less basic, but water alone cannot change an acid to a base or vice versa.

Question 5

Which of the following is a characteristic of a solution containing a strong base, such as sodium hydroxide (NaOH)?

  1. It partially dissociates, establishing an equilibrium between the base and its ions.
  2. It completely dissociates in solution to yield a high concentration of OH- ions. (correct answer)
  3. It readily donates protons to water, creating H3O+ ions.
  4. It is always a highly concentrated solution with a very low pH value.
Explanation: When you encounter questions about acids and bases, focus on the fundamental difference between strong and weak substances based on their dissociation behavior in water. Strong bases like sodium hydroxide (NaOH) are defined by their complete dissociation in aqueous solution. When NaOH dissolves in water, it breaks apart entirely: NaOH→Na++OH−\text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^-. This complete dissociation produces a high concentration of hydroxide ions (OH⁻), making the solution strongly basic. This is exactly what option B describes. Option A incorrectly describes weak bases, not strong ones. Weak bases like ammonia (NH₃) only partially dissociate and establish equilibrium between the molecular form and ions. Strong bases don't reach equilibrium—they dissociate completely. Option C confuses acids with bases. Substances that donate protons (H⁺) to form hydronium ions (H₃O⁺) are acids, not bases. Bases accept protons or donate hydroxide ions. Option D contains two errors. First, a strong base doesn't need to be highly concentrated—even dilute NaOH solutions are still strong bases because of complete dissociation. Second, strong bases have high pH values (above 7), not low ones. Low pH indicates high acidity. For HESI chemistry questions, remember that "strong" refers to the degree of dissociation, not concentration. Strong acids and bases dissociate completely, while weak ones only partially dissociate. This distinction is crucial for predicting solution behavior and pH.

Question 6

The human body maintains the pH of blood within a very narrow range of 7.35 to 7.45. A condition called alkalosis occurs if the blood pH rises above 7.45. Which of the following would be consistent with a state of alkalosis?

  1. An increase in the concentration of hydrogen ions [H+].
  2. A decrease in the concentration of hydrogen ions [H+]. (correct answer)
  3. An equal concentration of [H+] and hydroxide ions [OH-].
  4. A significant increase in the concentration of carbonic acid [H2CO3].
Explanation: When you encounter pH questions on the HESI, remember that pH and hydrogen ion concentration have an inverse relationship. Understanding this fundamental concept is crucial for analyzing acid-base balance in the body. Alkalosis occurs when blood pH rises above 7.45, making the blood more basic (less acidic). Since pH is defined as the negative logarithm of hydrogen ion concentration (pH=−log⁡[H+]pH = -\log[H^+]), when pH increases, the concentration of hydrogen ions must decrease. This inverse relationship means that fewer H⁺ ions are present in alkalotic blood, which is exactly what answer choice B describes. Let's examine why the other options are incorrect. Choice A suggests an increase in H⁺ concentration, which would actually lower the pH and cause acidosis, not alkalosis. Choice C describes a neutral solution where [H⁺] equals [OH⁻], which occurs at pH 7.0 - this is neither acidosis nor alkalosis, but normal neutrality. Choice D indicates increased carbonic acid concentration, which would release more H⁺ ions into the blood, lowering pH and causing acidosis rather than alkalosis. For HESI success, always remember the pH-hydrogen ion inverse relationship: high pH means low H⁺ (alkalosis), and low pH means high H⁺ (acidosis). When you see acid-base questions, immediately think about whether the condition described would increase or decrease hydrogen ions, then work backwards to determine the pH effect.

Question 7

During a titration, a chemist slowly adds a 0.1 M NaOH solution (a strong base) to a 0.1 M HCl solution (a strong acid). What will the pH of the resulting solution be at the equivalence point, where the moles of acid equal the moles of base?

  1. Less than 7
  2. Greater than 7
  3. Exactly 7 (correct answer)
  4. Exactly 0
Explanation: When you encounter acid-base titration questions, focus on what happens when equal molar amounts of acid and base react. This tests your understanding of neutralization reactions and the resulting pH. At the equivalence point in this titration, the strong acid HCl and strong base NaOH react completely in a 1:1 ratio: HCl+NaOH→NaCl+H2O\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}. Since both reactants have equal molarity (0.1 M) and equal molar amounts are present, they neutralize each other completely. The products are salt (NaCl) and water. Sodium chloride is the salt of a strong acid and strong base, making it a neutral salt that doesn't affect the pH of the solution. Therefore, the resulting solution has a pH of exactly 7. Answer choice (A) "Less than 7" would occur if you had excess acid remaining or if the salt formed were acidic, but neither applies here. Answer choice (B) "Greater than 7" would happen with excess base or a basic salt, which also doesn't occur in this strong acid-strong base titration. Answer choice (D) "Exactly 0" represents a fundamental misunderstanding—this would indicate an extremely acidic solution with [H+]=1 M[\text{H}^+] = 1 \text{ M}, not a neutralized solution. The correct answer is (C) exactly 7. Study tip: Remember that strong acid + strong base = neutral solution (pH 7) at equivalence. Only weak acid-strong base or strong acid-weak base titrations give pH ≠ 7 at equivalence due to salt hydrolysis.

Question 8

A patient is experiencing metabolic acidosis, a condition where the blood pH drops. The bicarbonate buffer system in the blood works to counteract this change. Which of the following describes the primary action of the bicarbonate buffer in this situation?

  1. The carbonic acid (H2CO3) acts as a weak acid to donate excess H+ ions.
  2. The bicarbonate ion (HCO3-) acts as a weak base to accept excess H+ ions. (correct answer)
  3. The buffer system completely neutralizes all excess acid by forming only water.
  4. The bicarbonate ion (HCO3-) releases hydroxide ions (OH-) to directly increase the pH.
Explanation: When you encounter acid-base balance questions on the HESI, focus on understanding how buffer systems maintain pH homeostasis. The bicarbonate buffer system is the body's most important extracellular buffer, consisting of carbonic acid (H₂CO₃) and bicarbonate ion (HCO₃⁻). In metabolic acidosis, excess hydrogen ions (H⁺) flood the bloodstream, dropping the pH below normal. The bicarbonate buffer system responds by having the bicarbonate ion (HCO₃⁻) act as a weak base, accepting these excess H⁺ ions to form carbonic acid (H₂CO₃). This reaction removes free hydrogen ions from solution, preventing the pH from dropping further. Choice A is backwards—in acidosis, you need to remove H⁺ ions, not add more. Carbonic acid would worsen the situation by releasing additional hydrogen ions. Choice C represents a common misconception; buffer systems resist pH changes but don't completely neutralize acids—they work within limits and maintain equilibrium. Choice D incorrectly describes the mechanism; bicarbonate doesn't release hydroxide ions directly. Instead, it accepts hydrogen ions, which indirectly helps stabilize pH. The correct answer is B because the bicarbonate ion functions as the base component of this buffer pair, accepting excess H⁺ ions during acidotic conditions. For HESI success, remember that buffer systems always work in pairs: one component handles excess acid (base accepts H⁺), while the other handles excess base (acid donates H⁺). Focus on which component is needed based on the pH disturbance direction.

Question 9

A nurse is handling a chemical spill of an unknown substance. The substance feels slippery to the touch and, when tested with litmus paper, turns it blue. These properties are characteristic of which type of substance?

  1. A strong acid
  2. An alkaline solution (correct answer)
  3. A neutral salt solution
  4. A nonpolar solvent
Explanation: Chemical safety questions on the HESI often test your ability to identify substances based on their physical and chemical properties. When you encounter an unknown chemical, specific characteristics can help you determine what type of substance you're dealing with. The key clues here are that the substance feels slippery and turns litmus paper blue. These are classic indicators of a basic (alkaline) solution. Bases have a characteristic slippery or soapy feel because they react with oils in your skin to form soap-like compounds. When litmus paper turns blue, it confirms the presence of a base, as bases have a pH greater than 7. Looking at the wrong answers: A) Strong acids would turn litmus paper red, not blue, and typically don't feel slippery but rather may cause a burning sensation. C) Neutral salt solutions have a pH of 7 and wouldn't change litmus paper color significantly—it would remain its original color. D) Nonpolar solvents like oils or hexane don't affect litmus paper at all since they don't ionize in water, and they typically feel oily rather than slippery in the soap-like way that bases do. Answer B is correct because alkaline solutions exhibit exactly these properties: they feel slippery due to their ability to saponify fats and oils, and they turn litmus paper blue due to their basic pH. For HESI chemical safety questions, remember this pattern: red litmus = acid, blue litmus = base, slippery feel = base. Always associate multiple properties together rather than relying on just one indicator.

Question 10

A medical laboratory technician measures the pH of a patient's blood sample and finds it to be 7.25. If the normal blood pH range is 7.35-7.45, and the patient's condition causes the hydrogen ion concentration to decrease by 20%, what will be the new pH value?

  1. The new pH will be approximately 7.30 because a 20% decrease in hydrogen ions causes a proportional pH increase
  2. The new pH will be approximately 7.20 because decreased hydrogen ion concentration paradoxically lowers pH in blood
  3. The new pH will be approximately 7.45 because the 20% hydrogen ion decrease shifts pH into normal range maximum
  4. The new pH will be approximately 7.35 because decreasing [H+] by 20% increases pH by 0.10 units logarithmically (correct answer)
Explanation: When you encounter pH calculations on the HESI, remember that pH and hydrogen ion concentration have an inverse logarithmic relationship. As hydrogen ions decrease, pH increases, but not in a simple linear fashion. Starting with a blood pH of 7.25, you can calculate the hydrogen ion concentration using the formula: [H+]=10−pH[H^+] = 10^{-pH}. This gives us [H+]=10−7.25=5.62×10−8[H^+] = 10^{-7.25} = 5.62 \times 10^{-8} M. When hydrogen ions decrease by 20%, the new concentration becomes 5.62×10−8×0.8=4.50×10−85.62 \times 10^{-8} \times 0.8 = 4.50 \times 10^{-8} M. Converting back to pH: pH=−log⁡(4.50×10−8)=7.35pH = -\log(4.50 \times 10^{-8}) = 7.35. This confirms that decreasing hydrogen ion concentration by 20% increases the pH by exactly 0.10 units due to the logarithmic relationship. Answer A incorrectly assumes a proportional relationship between hydrogen ion changes and pH changes, but pH operates on a logarithmic scale. Answer B contains a fundamental error—decreased hydrogen ion concentration always increases pH, never decreases it. Answer C miscalculates the magnitude of change; while the direction is correct, a 20% decrease in hydrogen ions from pH 7.25 cannot jump to 7.45. For HESI acid-base questions, always remember that pH changes follow logarithmic patterns, not linear ones. A useful rule: every 0.3 pH unit change represents a doubling or halving of hydrogen ion concentration. Understanding this logarithmic relationship will help you avoid common calculation traps on the exam.

Question 11

A student preparing buffer solutions needs to understand the relationship between pH and pOH. If a solution at 25°C has a pOH of 3.7, and the student dilutes this solution with pure water until the volume doubles, what will be the approximate new pH?

  1. The new pH will be approximately 10.0 because dilution decreases hydroxide concentration by half, changing pOH to 4.0 (correct answer)
  2. The new pH will be approximately 9.7 because dilution changes pOH to 4.0, making pH equal to 10.0
  3. The new pH will be approximately 10.6 because the original pH was 10.3 and dilution increases pH by 0.3 units
  4. The new pH will be approximately 20.6 because pH and pOH are inversely related and dilution doubles the effect
Explanation: Initial pOH = 3.7, so initial pH = 14.0 - 3.7 = 10.3. When diluted by factor of 2, [OH-] decreases by factor of 2, so pOH increases by log(2) = 0.3 to become 4.0. New pH = 14.0 - 4.0 = 10.0. Choice B incorrectly calculates the final pH. Choice C incorrectly assumes pH increases with dilution of a base. Choice D uses impossible pH values above 14.

Question 12

A laboratory solution has a hydroxide ion concentration of 2.0×10−42.0 \times 10^{-4} M at 25°C. A student needs to determine if this solution can neutralize 50 mL of 1.0×10−31.0 \times 10^{-3} M HCl. What volume of the hydroxide solution is required for complete neutralization?

  1. 25 mL of the hydroxide solution is needed because the acid concentration is twice as high as the base concentration
  2. 100 mL of the hydroxide solution is needed because the acid concentration is five times higher than the base concentration
  3. 250 mL of the hydroxide solution is needed because the acid has five times higher concentration requiring proportional volume adjustment (correct answer)
  4. 50 mL of the hydroxide solution is needed because equal volumes of acid and base always achieve neutralization regardless of concentration
Explanation: For neutralization: moles of H+ = moles of OH-. HCl provides (50 mL)(1.0 × 10^-3 M) = 5.0 × 10^-5 moles H+. The base provides (V mL)(2.0 × 10^-4 M) OH-. Setting equal: 5.0 × 10^-5 = V × 2.0 × 10^-4, so V = 250 mL. Choice A incorrectly calculates the concentration ratio. Choice B uses wrong concentration ratio. Choice D incorrectly assumes volume equality regardless of concentration.

Question 13

A patient's blood sample shows a pH of 7.35. If the hydrogen ion concentration increases by a factor of 2, what will be the approximate new pH value?

  1. The new pH will be approximately 6.70 because doubling hydrogen ions halves the pH value proportionally
  2. The new pH will be approximately 7.05 because the pH decreases by 0.30 units when hydrogen ions double (correct answer)
  3. The new pH will be approximately 7.65 because increased hydrogen ions paradoxically raise blood pH due to buffering
  4. The new pH will be approximately 14.70 because the relationship between pH and hydrogen ion concentration is inverse
Explanation: When hydrogen ion concentration doubles, the pH decreases by log₁₀(2) = 0.30 units. Starting from pH 7.35, the new pH = 7.35 - 0.30 = 7.05. Choice A incorrectly assumes a linear relationship between pH and [H+]. Choice C incorrectly suggests that increased [H+] raises pH. Choice D incorrectly applies the inverse relationship and would result in an impossible pH value above 14.

Question 14

A chemistry student is asked to identify an unknown solution. The solution tastes sour and reacts with zinc metal to produce hydrogen gas. These characteristics are typical of what kind of substance?

  1. An acid (correct answer)
  2. A base
  3. A pure solvent like water
  4. An ionic salt
Explanation: When you encounter questions about identifying chemical substances based on their properties, focus on the characteristic behaviors that distinguish acids, bases, and other compounds. The two key properties described here—sour taste and reaction with zinc to produce hydrogen gas—are classic identifying characteristics of acids. Acids have a distinctive sour taste (think lemon juice or vinegar) and react with metals like zinc according to the reaction: Zn+2H+→Zn2++H2\text{Zn} + 2\text{H}^+ \rightarrow \text{Zn}^{2+} + \text{H}_2. The hydrogen ions (H⁺) from the acid are what drive this gas-producing reaction. Looking at the wrong answers: (B) A base would taste bitter, not sour, and wouldn't produce hydrogen gas when mixed with zinc. Bases contain hydroxide ions (OH⁻) or accept protons, giving them completely different chemical behavior. (C) A pure solvent like water has a neutral pH around 7, tastes neither sour nor bitter, and doesn't react with zinc under normal conditions to produce gas. (D) An ionic salt might dissolve in water but typically doesn't taste sour or react with metals to generate hydrogen gas—salts are usually the products of acid-base reactions, not reactants with metals. For HESI chemistry questions, memorize these acid identifiers: sour taste, pH below 7, reaction with metals producing hydrogen gas, and ability to neutralize bases. These property-based identification questions are common, so practice connecting observable characteristics to substance types rather than just memorizing formulas.

Question 15

In the reversible reaction NH3+H2O⇌NH4++OH−\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-, how is water (H2O) functioning according to the Brønsted-Lowry definition?

  1. As an acid, because it donates a proton to NH3. (correct answer)
  2. As a base, because it accepts a proton from NH3.
  3. As a salt, because it is a product of a neutralization.
  4. As a neutral spectator, because it is the solvent.
Explanation: When you encounter acid-base reactions, the Brønsted-Lowry definition focuses on proton (H⁺) transfer: acids donate protons, while bases accept them. Look at what's actually happening to each molecule in the reaction. In this reaction, NH3+H2O⇌NH4++OH−\text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4^+ + \text{OH}^-, water starts as H₂O and becomes OH⁻. Notice that water has lost a hydrogen ion (proton) in the process. Since the Brønsted-Lowry definition states that acids are proton donors, water is functioning as an acid by giving up its proton to ammonia. Meanwhile, NH₃ accepts that proton to become NH₄⁺, making ammonia the base. Looking at the incorrect options: B suggests water accepts a proton from NH₃, but this is backwards—NH₃ doesn't have any protons to give up initially. C calls water a salt, but salts are ionic compounds formed from acid-base neutralization reactions, not participants that lose protons during the reaction. D incorrectly labels water as a neutral spectator, but water is actively participating by donating a proton, not just providing a medium for the reaction. The correct answer is A because water donates a proton to NH₃. Study tip: In Brønsted-Lowry problems, trace the hydrogen atoms carefully. The species that loses H⁺ is the acid, regardless of what you might expect from common knowledge. Water can act as either an acid or base depending on what it's reacting with—this is called being amphoteric.

Question 16

Normal rainwater is slightly acidic with a pH of about 5.6. Acid rain, however, can have a pH of 4.6. This one-unit drop in pH indicates that acid rain is...

  1. 10 times more acidic than normal rainwater. (correct answer)
  2. 1 time more acidic than normal rainwater.
  3. slightly less acidic than normal rainwater.
  4. 10 times less acidic than normal rainwater.
Explanation: When you encounter pH questions, remember that the pH scale is logarithmic, meaning each unit represents a 10-fold change in acidity. This is crucial for interpreting seemingly small numerical differences. To solve this problem, you need to calculate the difference in hydrogen ion concentration between the two pH values. The formula is: [H+]=10−pH[H^+] = 10^{-pH} For normal rainwater (pH 5.6): [H+]=10−5.6[H^+] = 10^{-5.6} For acid rain (pH 4.6): [H+]=10−4.6[H^+] = 10^{-4.6} To find how many times more acidic acid rain is, divide the acid rain concentration by normal rainwater concentration: 10−4.610−5.6=10−4.6−(−5.6)=101.0=10\frac{10^{-4.6}}{10^{-5.6}} = 10^{-4.6-(-5.6)} = 10^{1.0} = 10 This confirms that answer A is correct—acid rain is 10 times more acidic than normal rainwater. Answer B (1 time more acidic) incorrectly treats the pH scale as linear rather than logarithmic. Answer C (slightly less acidic) reverses the relationship entirely—a lower pH means higher acidity, not lower. Answer D (10 times less acidic) gets the magnitude right but the direction wrong, suggesting the student calculated correctly but misinterpreted which solution was more acidic. Remember this key principle: on the logarithmic pH scale, each whole number decrease represents a 10-fold increase in acidity. This pattern appears frequently in science sections, so practice converting between pH units and actual hydrogen ion concentrations.

Question 17

A solution is prepared with a hydroxide ion [OH-] concentration of 1×10−41 \times 10^{-4} M. Which of the following best describes this solution?

  1. It is an acidic solution with a pH of 4.
  2. It is a basic solution with a pH of 10. (correct answer)
  3. It is a basic solution with a pH of 4.
  4. It is an acidic solution with a pH of 10.
Explanation: When you encounter pH and pOH problems on the HESI, you need to understand the relationship between hydroxide ion concentration, hydrogen ion concentration, and the pH scale. At 25°C, water has the equilibrium constant Kw=[H+][OH−]=1×10−14K_w = [H^+][OH^-] = 1 \times 10^{-14}. Given that [OH−]=1×10−4[OH^-] = 1 \times 10^{-4} M, you can find the hydrogen ion concentration: [H+]=1×10−141×10−4=1×10−10[H^+] = \frac{1 \times 10^{-14}}{1 \times 10^{-4}} = 1 \times 10^{-10} M. The pH is calculated as pH=−log⁡[H+]=−log⁡(1×10−10)=10pH = -\log[H^+] = -\log(1 \times 10^{-10}) = 10. Since the pH is greater than 7, this solution is basic. Answer choice B correctly identifies this as a basic solution with pH 10. Answer choice A incorrectly states the solution is acidic with pH 4. This represents a fundamental misunderstanding—confusing the hydroxide concentration's exponent (-4) with the actual pH value. Answer choice C incorrectly calls it basic but assigns pH 4. While recognizing that high hydroxide concentration means basic conditions, it makes the same mathematical error as choice A. Answer choice D incorrectly labels the solution as acidic with pH 10. This contradicts basic chemistry principles since pH values above 7 are always basic, never acidic. Study tip: Remember that pH+pOH=14pH + pOH = 14 at 25°C. When given [OH−][OH^-], calculate pOH=−log⁡[OH−]pOH = -\log[OH^-], then subtract from 14 to get pH. If pOH<7pOH < 7, the solution is basic; if pOH>7pOH > 7, it's acidic.

Question 18

A patient's stomach fluid has a pH of 2. After taking an antacid, the pH of the fluid changes to 4. What is the change in the hydrogen ion concentration ([H+]) in the patient's stomach?

  1. The [H+] decreased by a factor of 2.
  2. The [H+] increased by a factor of 100.
  3. The [H+] decreased by a factor of 100. (correct answer)
  4. The [H+] decreased by half.
Explanation: When you encounter pH problems on the HESI exam, remember that pH is a logarithmic scale where each unit represents a 10-fold change in hydrogen ion concentration. The relationship is: [H+]=10−pH[H^+] = 10^{-pH} To find the change in hydrogen ion concentration, calculate the [H+] at both pH values. At pH 2: [H+]=10−2[H^+] = 10^{-2}. At pH 4: [H+]=10−4[H^+] = 10^{-4}. To determine the factor of change, divide the initial concentration by the final concentration: 10−210−4=10−2−(−4)=102=100\frac{10^{-2}}{10^{-4}} = 10^{-2-(-4)} = 10^2 = 100 This means the hydrogen ion concentration decreased by a factor of 100, making answer C correct. Let's examine why the other options are wrong. Answer A suggests the [H+] decreased by a factor of 2, which would be true if pH changed by about 0.3 units, not 2 full units. Answer B states the [H+] increased by a factor of 100, but since pH increased (became less acidic), the [H+] must have decreased, not increased. Answer D claims the [H+] decreased by half, which again would require only a small pH change of about 0.3 units. Study tip: For HESI pH questions, remember the "rule of 10s" - each pH unit change represents a 10-fold change in [H+]. When pH increases by 2 units, [H+] decreases by 102=10010^2 = 100 times. This logarithmic relationship is frequently tested in acid-base problems.

Question 19

According to the Arrhenius definition, what defines a substance as a base?

  1. It accepts a proton from another substance.
  2. It donates a proton to another substance.
  3. It produces hydroxide ions (OH-) when dissolved in water. (correct answer)
  4. It increases the hydrogen ion (H+) concentration in water.
Explanation: When you encounter questions about acid-base definitions, you need to distinguish between the three major theories: Arrhenius, Brønsted-Lowry, and Lewis. Each defines acids and bases differently, so identifying which theory the question asks about is crucial. The Arrhenius definition is the most straightforward and focuses specifically on what happens when substances dissolve in water. According to Svante Arrhenius, a base is any substance that produces hydroxide ions (OH⁻) when dissolved in water. For example, when sodium hydroxide (NaOH) dissolves in water, it releases OH⁻ ions, making it an Arrhenius base. This makes option C correct. Let's examine why the other choices are wrong. Option A describes the Brønsted-Lowry definition of a base (proton acceptor), not the Arrhenius definition. Option B describes a Brønsted-Lowry acid (proton donor), which is the opposite of what we're looking for. Option D describes an Arrhenius acid, which increases H⁺ concentration in water—again, the opposite of a base. The key distinction is that Arrhenius theory is water-specific and ion-focused, while Brønsted-Lowry theory is about proton transfer and works in any solvent. Remember this pattern: on the HESI, acid-base questions often test whether you can match the correct definition to the correct theory. Always read carefully to see which scientist or theory is mentioned, as this determines which definition applies.

Question 20

Solution X has a pH of 9, and Solution Y has a pH of 12. Which statement correctly compares the two solutions?

  1. Solution Y is 3 times more basic than Solution X.
  2. Solution X is 1,000 times more acidic than Solution Y.
  3. Solution Y is 1,000 times more basic than Solution X. (correct answer)
  4. Solution Y is 30 times more basic than Solution X.
Explanation: When you encounter pH comparison questions, remember that the pH scale is logarithmic, meaning each unit represents a 10-fold change in hydrogen ion concentration. This logarithmic relationship is crucial for calculating the actual differences between solutions. To compare these solutions, you need to calculate the difference in hydrogen ion concentration. Solution X (pH 9) has a hydrogen ion concentration of 10−910^{-9} M, while Solution Y (pH 12) has 10−1210^{-12} M. Since basicity is inversely related to hydrogen ion concentration, Solution Y is 10−910−12=103=1,000\frac{10^{-9}}{10^{-12}} = 10^3 = 1,000 times more basic than Solution X. Option A incorrectly suggests a simple arithmetic relationship (12 - 9 = 3), but pH doesn't work this way. The 3-unit difference represents a logarithmic change, not a linear one. Option B makes two errors: it focuses on acidity rather than basicity, and while the 1,000-fold difference is numerically correct, it's backwards—Solution X is actually 1,000 times more acidic than Solution Y, not the other way around. Option D falls into the same trap as A, treating the pH scale as if it were linear (3 × 10 = 30). For HESI success, always remember that pH questions test your understanding of logarithmic scales. When calculating differences between pH values, convert to the actual ion concentrations using powers of 10, then compare. Each pH unit represents a 10-fold change, so a 3-unit difference means 103=1,00010^3 = 1,000 times difference in concentration.