Acid-Base Chemistry - MCAT Chemical and Physical Foundations of Biological Systems
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What volume of 0.375M H2SO4 is needed to fully neutralize 0.5L of 0.125M NaOH?
What volume of 0.375M H2SO4 is needed to fully neutralize 0.5L of 0.125M NaOH?
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This question requires use of the simple titration equation M1V1 = M2V2. The key is to identify that sulfuric acid has two equivalents of acidic hydrogens while NaOH has only one hydroxide equivalent. All wrong answer choices result from making this mistake or other calculation errors.
This question requires use of the simple titration equation M1V1 = M2V2. The key is to identify that sulfuric acid has two equivalents of acidic hydrogens while NaOH has only one hydroxide equivalent. All wrong answer choices result from making this mistake or other calculation errors.
HCN dissociates based on the following reaction.
The Ka for hydrogen cyanide is 
.
Suppose that a solution with a pH of 4.5 has 2M HCN added. Which of the following values will change?
HCN dissociates based on the following reaction.
The Ka for hydrogen cyanide is
Suppose that a solution with a pH of 4.5 has 2M HCN added. Which of the following values will change?
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Remember that equilibrium constants are not affected by the concentrations of the reactants and products. Since an acid is being added to the solution, the pH of the solution will be affected. This means that the pOH will be affected as well.
Remember that equilibrium constants are not affected by the concentrations of the reactants and products. Since an acid is being added to the solution, the pH of the solution will be affected. This means that the pOH will be affected as well.
A weak acid is slowly titrated with a strong base. Where on the titration curve would the solution be the most well-buffered?
A weak acid is slowly titrated with a strong base. Where on the titration curve would the solution be the most well-buffered?
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At the half equivalence point, the concentration of acid in the solution is equal to the concentration of the conjugate base in solution. At this point, the graph shows a line that is near horizontal. This means that base or acid could be added and the pH of the solution would change very slowly.
Remember that pH is on the y-axis of the titration curve; thus a near-horizontal line will signify a region where pH is most stable. At the half equivalence point, pH = pKa.
At the half equivalence point, the concentration of acid in the solution is equal to the concentration of the conjugate base in solution. At this point, the graph shows a line that is near horizontal. This means that base or acid could be added and the pH of the solution would change very slowly.
Remember that pH is on the y-axis of the titration curve; thus a near-horizontal line will signify a region where pH is most stable. At the half equivalence point, pH = pKa.
Which of the following would be most useful as a buffer?
Which of the following would be most useful as a buffer?
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A buffer must contain either a weak base and its salt or a weak acid and its salt. A mixture of ammonia and ammonium chloride is an example of the first case, since ammonia, NH3, is a weak base and ammonium chloride, NH4Cl, contains its salt.
Though autoionization of water produces small amounts of H3O+ and OH-, each conjugate salts of H2O, they exist in such small amounts as to make any buffering effects negligible.
A buffer must contain either a weak base and its salt or a weak acid and its salt. A mixture of ammonia and ammonium chloride is an example of the first case, since ammonia, NH3, is a weak base and ammonium chloride, NH4Cl, contains its salt.
Though autoionization of water produces small amounts of H3O+ and OH-, each conjugate salts of H2O, they exist in such small amounts as to make any buffering effects negligible.
Which of the following is the strongest acid?
Which of the following is the strongest acid?
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Hydroiodic acid (HI) is the strongest acid listed. Charge density decreases as the atomic size gets larger, thus stabilizing the charge when a hydrogen is given off. Hydroflouric acid (HF) is a relatively weak acid because electronegative elements hold on to their valence electrons more tightly, and are thus less likely to dissociate.
Hydroiodic acid (HI) is the strongest acid listed. Charge density decreases as the atomic size gets larger, thus stabilizing the charge when a hydrogen is given off. Hydroflouric acid (HF) is a relatively weak acid because electronegative elements hold on to their valence electrons more tightly, and are thus less likely to dissociate.
Acids and bases can be described in three principal ways. The Arrhenius definition is the most restrictive. It limits acids and bases to species that donate protons and hydroxide ions in solution, respectively. Examples of such acids include HCl and HBr, while KOH and NaOH are examples of bases. When in aqueous solution, these acids proceed to an equilibrium state through a dissociation reaction.
All of the bases proceed in a similar fashion.
The Brønsted-Lowry definition of an acid is a more inclusive approach. All Arrhenius acids and bases are also Brønsted-Lowry acids and bases, but the converse is not true. Brønsted-Lowry acids still reach equilibrium through the same dissociation reaction as Arrhenius acids, but the acid character is defined by different parameters. The Brønsted-Lowry definition considers bases to be hydroxide donors, like the Arrhenius definition, but also includes conjugate bases such as the A- in the above reaction. In the reverse reaction, A- accepts the proton to regenerate HA. The Brønsted-Lowry definition thus defines bases as proton acceptors, and acids as proton donors.
Reducing agents, such as 
, are used to donate hydrogen ions to chemical species. 
will dissociate in solution to form hydrogen ions, which then reduce the target compound.
Which of the following must be true of 
when it is used as a reducing agent?
Acids and bases can be described in three principal ways. The Arrhenius definition is the most restrictive. It limits acids and bases to species that donate protons and hydroxide ions in solution, respectively. Examples of such acids include HCl and HBr, while KOH and NaOH are examples of bases. When in aqueous solution, these acids proceed to an equilibrium state through a dissociation reaction.
All of the bases proceed in a similar fashion.
The Brønsted-Lowry definition of an acid is a more inclusive approach. All Arrhenius acids and bases are also Brønsted-Lowry acids and bases, but the converse is not true. Brønsted-Lowry acids still reach equilibrium through the same dissociation reaction as Arrhenius acids, but the acid character is defined by different parameters. The Brønsted-Lowry definition considers bases to be hydroxide donors, like the Arrhenius definition, but also includes conjugate bases such as the A- in the above reaction. In the reverse reaction, A- accepts the proton to regenerate HA. The Brønsted-Lowry definition thus defines bases as proton acceptors, and acids as proton donors.
Reducing agents, such as
Which of the following must be true of
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is not an acid when it is used as a reducing agent. This can confuse students because, 
is in fact donating a hydrogen ion to a compound that is being reduced; however, we are reducing something, and thus we are adding electrons. The only way that 
could be an effective reducing agent is if it carried electrons on its donated hydrogen.
As a result, we are donating hydride (
) ions, and not protons. Acids are exclusively those species that donate protons in solution, not hydrides.
As a result, we are donating hydride (
Which of the following is considered a weak base?
Which of the following is considered a weak base?
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Strong bases are those that completely dissociate in water. All group I hydroxides are considered strong bases, including sodium hydroxide, potassium hydroxide, and lithium hydroxide. Some group II hydroxides are also strong bases, including calcium hydroxide, barium hydroxide, and strontium hydroxide.
Ammonia is considered a weak base and is frequently used as a buffer in solution with ammonium, its conjugate acid.
Strong bases are those that completely dissociate in water. All group I hydroxides are considered strong bases, including sodium hydroxide, potassium hydroxide, and lithium hydroxide. Some group II hydroxides are also strong bases, including calcium hydroxide, barium hydroxide, and strontium hydroxide.
Ammonia is considered a weak base and is frequently used as a buffer in solution with ammonium, its conjugate acid.
What is the pKa of acetic acid? (Ka = 1.8 * 10–5)
What is the pKa of acetic acid? (Ka = 1.8 * 10–5)
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We know that pKa is equal to –log(Ka). Thus, pKa of acetic acid is –log(1.8 * 10–5). This is not an easy problem to solve in your head, but there is a trick.
We know that 1 * 10–4 > 1.8 * 10–5 > 1 * 10–5, and we know that –log(1 * 10–4) = 4 and –log(1 * 10–5) = 5. Now we can conclude that our pKa is somewhere between 4 and 5.
Two answer choices fall in this range: 4.2 and 4.7.
1.8 * 10–5 is closer to 1 * 10–5 than it is to 1 * 10–4, so we can pick the answer closer to 5 than to 4 : 4.7.
We know that pKa is equal to –log(Ka). Thus, pKa of acetic acid is –log(1.8 * 10–5). This is not an easy problem to solve in your head, but there is a trick.
We know that 1 * 10–4 > 1.8 * 10–5 > 1 * 10–5, and we know that –log(1 * 10–4) = 4 and –log(1 * 10–5) = 5. Now we can conclude that our pKa is somewhere between 4 and 5.
Two answer choices fall in this range: 4.2 and 4.7.
1.8 * 10–5 is closer to 1 * 10–5 than it is to 1 * 10–4, so we can pick the answer closer to 5 than to 4 : 4.7.
Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.
CO2 + H2O 
H2CO3
H2CO3 
HCO3- + H+
HCO3- may be considered a(n) .
Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.
CO2 + H2O
H2CO3
HCO3- may be considered a(n) .
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First, we need to determine from both the information presented in the paragraph and the chemical equation what HCO3- is doing. The paragraph tells us “HCO3- also \[serves\] to control the pH of the blood.” This is the definition of a buffer. A buffer is able to mitigate the addition of acid or base to a solution, or in this case, blood, by being deprotonated or protonated. For example, if the blood becomes too acidic, H+ will recombine with HCO3- to form H2CO3, thus stabilizing the pH. Alternatively, if the pH becomes too basic, H2CO3 will dissociate, releasing H+ into solution.
First, we need to determine from both the information presented in the paragraph and the chemical equation what HCO3- is doing. The paragraph tells us “HCO3- also \[serves\] to control the pH of the blood.” This is the definition of a buffer. A buffer is able to mitigate the addition of acid or base to a solution, or in this case, blood, by being deprotonated or protonated. For example, if the blood becomes too acidic, H+ will recombine with HCO3- to form H2CO3, thus stabilizing the pH. Alternatively, if the pH becomes too basic, H2CO3 will dissociate, releasing H+ into solution.
HCN dissociates based on the following reaction:
The Ka for hydrogen cyanide is 
.
Two moles of HCN are mixed with one mole of NaCN, resulting in 1L of solution. What is the pH of the solution?
HCN dissociates based on the following reaction:
The Ka for hydrogen cyanide is
Two moles of HCN are mixed with one mole of NaCN, resulting in 1L of solution. What is the pH of the solution?
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Whenever a concentration of the conjugate base is initially present in the solution, we say that the solution has been buffered. NaCN will completely dissolve in solution, meaning that there will be 1M of CN- ions in the solution initially (one mole of NaCN in 1L of soultion equals 1M). With this in mind, the new equilibrium equation for the acid's dissociation is as follows:
The initial concentrations of CN- and HCN are given in the question as 1M and 2M, respectively. The initial concentration of H+ is zero. As HCN begine to dissociate, H+ and CN- will each increase by X amount, while HCN will decrease by X amount.
Since 1M and 2M are much larger numbers than the value for X, the X values next to these numbers can be omitted from the equation.
By plugging this value into the equation 
, we determine that the pH of the solution is 8.9. (Remember that X is equal to the amount of increase in H+ ions).
This may seem wrong, because HCN is an acid, and we are used to acids resulting in solutions with pH values less than 7; however, HCN is a weak acid, and a large concentration of its conjugate base can result in a basic solution.
Whenever a concentration of the conjugate base is initially present in the solution, we say that the solution has been buffered. NaCN will completely dissolve in solution, meaning that there will be 1M of CN- ions in the solution initially (one mole of NaCN in 1L of soultion equals 1M). With this in mind, the new equilibrium equation for the acid's dissociation is as follows:
The initial concentrations of CN- and HCN are given in the question as 1M and 2M, respectively. The initial concentration of H+ is zero. As HCN begine to dissociate, H+ and CN- will each increase by X amount, while HCN will decrease by X amount.
Since 1M and 2M are much larger numbers than the value for X, the X values next to these numbers can be omitted from the equation.
By plugging this value into the equation
This may seem wrong, because HCN is an acid, and we are used to acids resulting in solutions with pH values less than 7; however, HCN is a weak acid, and a large concentration of its conjugate base can result in a basic solution.
The pH of a buffered solution is 
. What is the approximate ratio of the concentration of acid to conjugate base if the 
of the acid is 
?
The pH of a buffered solution is
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In a buffered solution, when the concentrations of acid and conjugate base are equal, we know the 
.
This is derived from the Henderson-Hasselbalch equation: 
.
When concentrations are equal, the log of 
is 
, and 
.
In our question, the pH is approximately one unit greater than the 
.
Because of this, we know that the log of the two concentrations must be equal to one.
The log of 
is 
, and therefore the conjugate base must be ten times greater than the acid; therefore the ratio of acid to base is approximately 
.
We can quickly narrow this question to two possible answers, since the buffered solution is more basic than the 
, and thus we would expect there to be a greater concentration of base than acid in the solution.
In a buffered solution, when the concentrations of acid and conjugate base are equal, we know the
This is derived from the Henderson-Hasselbalch equation:
When concentrations are equal, the log of
In our question, the pH is approximately one unit greater than the
Because of this, we know that the log of the two concentrations must be equal to one.
The log of
We can quickly narrow this question to two possible answers, since the buffered solution is more basic than the
Acids and bases can be described in three principal ways. The Arrhenius definition is the most restrictive. It limits acids and bases to species that donate protons and hydroxide ions in solution, respectively. Examples of such acids include HCl and HBr, while KOH and NaOH are examples of bases. When in aqueous solution, these acids proceed to an equilibrium state through a dissociation reaction.
All of the bases proceed in a similar fashion.
The Brønsted-Lowry definition of an acid is a more inclusive approach. All Arrhenius acids and bases are also Brønsted-Lowry acids and bases, but the converse is not true. Brønsted-Lowry acids still reach equilibrium through the same dissociation reaction as Arrhenius acids, but the acid character is defined by different parameters. The Brønsted-Lowry definition considers bases to be hydroxide donors, like the Arrhenius definition, but also includes conjugate bases such as the A- in the above reaction. In the reverse reaction, A- accepts the proton to regenerate HA. The Brønsted-Lowry definition thus defines bases as proton acceptors, and acids as proton donors.
A scientist is making a buffer to maintain the pH of a solution at about 6. Which of the following describes the best composition of the solution?
Acids and bases can be described in three principal ways. The Arrhenius definition is the most restrictive. It limits acids and bases to species that donate protons and hydroxide ions in solution, respectively. Examples of such acids include HCl and HBr, while KOH and NaOH are examples of bases. When in aqueous solution, these acids proceed to an equilibrium state through a dissociation reaction.
All of the bases proceed in a similar fashion.
The Brønsted-Lowry definition of an acid is a more inclusive approach. All Arrhenius acids and bases are also Brønsted-Lowry acids and bases, but the converse is not true. Brønsted-Lowry acids still reach equilibrium through the same dissociation reaction as Arrhenius acids, but the acid character is defined by different parameters. The Brønsted-Lowry definition considers bases to be hydroxide donors, like the Arrhenius definition, but also includes conjugate bases such as the A- in the above reaction. In the reverse reaction, A- accepts the proton to regenerate HA. The Brønsted-Lowry definition thus defines bases as proton acceptors, and acids as proton donors.
A scientist is making a buffer to maintain the pH of a solution at about 6. Which of the following describes the best composition of the solution?
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A buffer is best made of equal and copious amounts of an acid and its conjugate base. The acid must have a pKa as close as possible to the pH desired. For this question, the desired pH is 6. The best option will have a pKa of about 5.9.
Adding larger amounts of the acid than the base will skew the equilibrium in favor of the acid, increasing the overall proton concentration, and limiting the buffer ability to absorb more proton addition without changing pH significantly.
A buffer is best made of equal and copious amounts of an acid and its conjugate base. The acid must have a pKa as close as possible to the pH desired. For this question, the desired pH is 6. The best option will have a pKa of about 5.9.
Adding larger amounts of the acid than the base will skew the equilibrium in favor of the acid, increasing the overall proton concentration, and limiting the buffer ability to absorb more proton addition without changing pH significantly.
Which of the following would be a good buffer solution?
Which of the following would be a good buffer solution?
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A good buffer solution consists of a weak acid with its conjugate base, or a weak base and its conjugate acid. A strong acid or base will never be a good buffer. We will go through each answer choice in detail.
Sulfuric acid is a strong acid and cannot be used in a buffer.
Neither sodium chloride, nor calcium chloride constitute a weak acid or weak base. Both of these are simple salts and will not be able to create a buffer solution.
Hydrochloric acid is a strong acid and cannot be used in a buffer.
In solution, these salts will produce sulfate ions and hydrogen sulfate ions. Hydrogen sulfate is a weak acid and sulfate is its conjugate base. These ions will form a buffer solution.
A good buffer solution consists of a weak acid with its conjugate base, or a weak base and its conjugate acid. A strong acid or base will never be a good buffer. We will go through each answer choice in detail.
Which of the following pairs cannot constitute a buffer system?
Which of the following pairs cannot constitute a buffer system?
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A buffer system is composed of one of the following scenarios:
1. A weak base with its conjugate acid.
2. A weak acid with its conjugate base.
The only option that does not qualify is the nitrate, nitrite pair. A difference of one oxygen does not make a buffer system.
A buffer system is composed of one of the following scenarios:
1. A weak base with its conjugate acid.
2. A weak acid with its conjugate base.
The only option that does not qualify is the nitrate, nitrite pair. A difference of one oxygen does not make a buffer system.
None of the questions in this set require the use of a calculator. Math problems are intended to mimic the level of math intensity that you will see on the MCAT exam.
A Bronsted-Lowry acid will be able to .
None of the questions in this set require the use of a calculator. Math problems are intended to mimic the level of math intensity that you will see on the MCAT exam.
A Bronsted-Lowry acid will be able to .
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The Bronsted-Lowry definition of an acid is one of three acid definitions that can be used on the MCAT. The three definitions are as follows:
Arrhenius acid: increases H+ concentration in water (releases an H+)
Bronsted-Lowry acid: donates a proton to a Bronsted base (Bronsted bases accept protons)
Lewis acid: accepts a pair of electrons from a Lewis base (Lewis bases donate electrons)
An easy way to remember the difference between Bronsted-Lowry and Lewis is that the "e" in Lewis comes before the "e" in Bronsted; therefore the Lewis definition has to do with electrons.
The Bronsted-Lowry definition of an acid is one of three acid definitions that can be used on the MCAT. The three definitions are as follows:
Arrhenius acid: increases H+ concentration in water (releases an H+)
Bronsted-Lowry acid: donates a proton to a Bronsted base (Bronsted bases accept protons)
Lewis acid: accepts a pair of electrons from a Lewis base (Lewis bases donate electrons)
An easy way to remember the difference between Bronsted-Lowry and Lewis is that the "e" in Lewis comes before the "e" in Bronsted; therefore the Lewis definition has to do with electrons.
Which of the following is not a Lewis acid?
Which of the following is not a Lewis acid?
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The definition of a Lewis acid is an electron acceptor. Try drawing the Lewis dot structures for these compounds, and you will find that NH4+, Na+, and Al3+ are missing electrons. They each have an empty orbital, allowing them to accept electrons.
BF3 is not an ion; however, we know that boron only has 3 valence electrons, which means that even when they are all bound to fluorine the molecule does not satisfy the octet rule. BF3 only has 6 valence electrons around boron, and can accept another electron pair to get to the octet state.
CaO is a Lewis base. In the Lewis dot structure, we can see that the oxygen molecule has two lone pairs that it can donate to other molecules, making it an electron donor.
The definition of a Lewis acid is an electron acceptor. Try drawing the Lewis dot structures for these compounds, and you will find that NH4+, Na+, and Al3+ are missing electrons. They each have an empty orbital, allowing them to accept electrons.
BF3 is not an ion; however, we know that boron only has 3 valence electrons, which means that even when they are all bound to fluorine the molecule does not satisfy the octet rule. BF3 only has 6 valence electrons around boron, and can accept another electron pair to get to the octet state.
CaO is a Lewis base. In the Lewis dot structure, we can see that the oxygen molecule has two lone pairs that it can donate to other molecules, making it an electron donor.
Which of the following is a weak base?
Which of the following is a weak base?
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Group 1 hydroxides, group 1 oxides, Ba(OH)2, Sr(OH)2, Ca(OH)2, and metal amides are the only strong bases. Everything else is considered a weak base.
Group 1 hydroxides, group 1 oxides, Ba(OH)2, Sr(OH)2, Ca(OH)2, and metal amides are the only strong bases. Everything else is considered a weak base.
Which of the following will dissolve completely in water?
Which of the following will dissolve completely in water?
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The question is asking which of the following is a strong acid. Of the following, the only strong acid provided is perchloric acid, HClO4. NH3 is a weak base; HCOOH, C6H5COOH, and HCN are all weak acids. A strong acid is one that dissociates completely in water.
The strong acids that you must know for the MCAT are:
Hydroiodic acid (HI)
Hydrobromic acid (HBr)
Hydrochloric acid (HCl)
Perchloric acid (HClO4 )
Sulfuric acid (H2SO4)
and Nitric acid (HNO3)
The question is asking which of the following is a strong acid. Of the following, the only strong acid provided is perchloric acid, HClO4. NH3 is a weak base; HCOOH, C6H5COOH, and HCN are all weak acids. A strong acid is one that dissociates completely in water.
The strong acids that you must know for the MCAT are:
Hydroiodic acid (HI)
Hydrobromic acid (HBr)
Hydrochloric acid (HCl)
Perchloric acid (HClO4 )
Sulfuric acid (H2SO4)
and Nitric acid (HNO3)
Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.
CO2 + H2O 
H2CO3
H2CO3 
HCO3- + H+
HCO3- is the of H2CO3.
Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.
CO2 + H2O
H2CO3
HCO3- is the of H2CO3.
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First, we need to determine what type of reaction HCO3- is participating in. We can see that a H+ is being produced in the forward reaction, and consumed in the reserve reaction; thus, we are looking at an acid-base reaction. H2CO3 has lost a H+, making it a Arrhenius acid. HCO3- is the conjugate base of the acid H2CO3.
First, we need to determine what type of reaction HCO3- is participating in. We can see that a H+ is being produced in the forward reaction, and consumed in the reserve reaction; thus, we are looking at an acid-base reaction. H2CO3 has lost a H+, making it a Arrhenius acid. HCO3- is the conjugate base of the acid H2CO3.
Which of the following describes a Brønsted-Lowry acid?
Which of the following describes a Brønsted-Lowry acid?
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There are three definitions for an acid that are commonly used: an Arrhenius acid, a Brønsted-Lowry acid, and a Lewis acid. Arrhenius acids are any substances that create hydrogen ions in solution, and Lewis acids are any substances that accept an electron pair. A Brønsted-Lowry acid is defined as any substance that donates a proton.
There are three definitions for an acid that are commonly used: an Arrhenius acid, a Brønsted-Lowry acid, and a Lewis acid. Arrhenius acids are any substances that create hydrogen ions in solution, and Lewis acids are any substances that accept an electron pair. A Brønsted-Lowry acid is defined as any substance that donates a proton.