Acid-Base Chemistry - GRE Subject Test: Chemistry
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100mL of an unknown solution of NaOH is titrated with 3M HCl until neutralized. The resulting solution is evaporated, and 3.0g of white crystal are recovered. What was the concentration of the NaOH solution?
100mL of an unknown solution of NaOH is titrated with 3M HCl until neutralized. The resulting solution is evaporated, and 3.0g of white crystal are recovered. What was the concentration of the NaOH solution?
In the neutralization reaction between NaOH and HCl, NaCl salt is formed. When the solution is evaporated, this salt is left behind.
3.0g of NaCl is equivalent to 0.05mol NaCl. Since the titration is between a strong acid and a strong base, all of the NaOH in the original solution is converted to NaCl in a one-to-one ratio, meaning that mol NaCl = mol NaOH.
We now know that there was 0.05mol NaOH in the 100mL solution, so the concentration must have been 
.
In the neutralization reaction between NaOH and HCl, NaCl salt is formed. When the solution is evaporated, this salt is left behind.
3.0g of NaCl is equivalent to 0.05mol NaCl. Since the titration is between a strong acid and a strong base, all of the NaOH in the original solution is converted to NaCl in a one-to-one ratio, meaning that mol NaCl = mol NaOH.
We now know that there was 0.05mol NaOH in the 100mL solution, so the concentration must have been
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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?
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.
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Consider the following reaction:
Which of the following changes will increase the pH of this solution?
Consider the following reaction:
Which of the following changes will increase the pH of this solution?
To answer this question you need to use Le Chatelier’s principle. Adding sodium acetate to the solution will cause it to dissociate as follows:
The dissociation reaction will produce more acetate ions. According to Le Chatelier’s principle, the increase in acetate ions will shift the equilibrium of the reaction (given in the question) to the left. This means that 
and 
will be utilized to form 
. This will cause a decrease in the amount of hydronium ions in solution. Recall that pH is increased when the concentration of hydrogen ions (or hydronium ions, 
) is decreased; therefore, adding sodium acetate will increase the pH of the solution.
Increasing acetic acid concentration will shift the equilibrium to the right and produce more hydronium ions, thereby decreasing the pH. Recall that you can never change the pKa of an acid. The pKa of acetic acid is around 4.75, and it cannot be altered. Le Chatelier’s principle only applies when there is a change in amount of aqueous or gaseous substances; liquid and solid substances will not shift the equilibrium. Changing the volume of liquid water will not change the concentration of hydronium ions.
To answer this question you need to use Le Chatelier’s principle. Adding sodium acetate to the solution will cause it to dissociate as follows:
The dissociation reaction will produce more acetate ions. According to Le Chatelier’s principle, the increase in acetate ions will shift the equilibrium of the reaction (given in the question) to the left. This means that
Increasing acetic acid concentration will shift the equilibrium to the right and produce more hydronium ions, thereby decreasing the pH. Recall that you can never change the pKa of an acid. The pKa of acetic acid is around 4.75, and it cannot be altered. Le Chatelier’s principle only applies when there is a change in amount of aqueous or gaseous substances; liquid and solid substances will not shift the equilibrium. Changing the volume of liquid water will not change the concentration of hydronium ions.
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Which of the following acids is polyprotic?
Which of the following acids is polyprotic?
Sulfuric acid (
) is considered a polyprotic acid because it has two ionizable protons in its molecular formula. The protons dissociate in an aqueous solution according to the acid-base equilibria below:
All the other acids listed in the answer choices are monoprotic.
Sulfuric acid (
All the other acids listed in the answer choices are monoprotic.
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Which of the following acids is considered polyprotic?
Which of the following acids is considered polyprotic?
Carbonic acid (
) is considered a polyprotic acid because it has two ionizable protons (
atoms) in its molecular formula. The protons dissociate in an aqueous solution according to the acid-base equilibria below:
The other acids are all monoprotic acids.
Carbonic acid (
The other acids are all monoprotic acids.
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Considering the Ka for 
is 
, what is the Kb for 
?
Considering the Ka for
The equilibrium governing the dissolution of 
in water is:
is the conjugate acid of 
. In other words, 
is the conjugate base of 
.
Using the relationship, 
, we can calculate the Kb.
Rearrange the equation and solve:
The equilibrium governing the dissolution of
Using the relationship,
Rearrange the equation and solve:
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Considering the Ka for 
is 
, what is the Kb for 
?
Considering the Ka for
The equilibrium governing the dissolution of 
in water is:
is the conjugate acid of 
. In other words, 
is the conjugate base of 
.
Using the relationship, 
, we can calculate the Kb.
Rearrange the equation and solve:
The equilibrium governing the dissolution of
Using the relationship,
Rearrange the equation and solve:
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Based on the equilibrium shown, what does 
act as?
Based on the equilibrium shown, what does
A base is a substance that can accept a proton. The conjugate base of an acid is formed when the acid donates a proton. In this case, 
is the conjugate base to the acid 
. This is because 
donates a hydrogen ion to the organic molecule to form 
, the conjugate base.
A base is a substance that can accept a proton. The conjugate base of an acid is formed when the acid donates a proton. In this case,
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Which of the following is the weakest acid?
Which of the following is the weakest acid?
(hydrofluoric acid) is the weakest acid. Fluoride ion is the most electronegative ion. Among the other halogens, its atomic radius is smaller, and therefore bonds more strongly with hydrogen and therefore does not completely dissociate in solution as compared to 
, 
, and 
. Perchloric acid is a strong acid and dissociates completely in solution.
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Based on the above balanced equation for a neutralization reaction, what is the 
concentration of a 
solution if 
of 
is needed to neutralize a 
solution of 
?
Based on the above balanced equation for a neutralization reaction, what is the
We need to convert milliliters to liters:
We need to determine the moles of 
using dimensional analysis the concentration as a conversion factor:
Based on the chemical equation given:
We can used the relationship of moles of 
and moles of 
as a conversion factor to determine the moles of 
:
Concentration in molarity can be calculated using the following formula:
Let's convert liters of 
to the mL:
Therefore, the concentration of 
is:
We need to convert milliliters to liters:
We need to determine the moles of
Based on the chemical equation given:
We can used the relationship of moles of
Concentration in molarity can be calculated using the following formula:
Let's convert liters of
Therefore, the concentration of
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Based on the above balanced equation for a neutralization reaction, how many moles of 
is formed if 
of 
is needed to neutralize a solution of ?
Based on the above balanced equation for a neutralization reaction, how many moles of
We need to convert milliliters of 
to liters:
We need to determine the moles of 
using dimensional analysis and the concentration as a conversion factor:
Based on the chemical equation given:
We can use the relationship of moles of 
and moles of as a conversion factor to determine the moles of :
We need to convert milliliters of
We need to determine the moles of
Based on the chemical equation given:
We can use the relationship of moles of
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What is the type of reaction given?
What is the type of reaction given?
The reaction given is called a neutralization reaction because the acid and base components react to counterbalance each other making them chemically neutral. This type of reaction occurs between an acid and base to form a water and a salt. A neutralization reaction between a strong acid and a strong base react to form a neutral solution of pH 7.
The reaction given is called a neutralization reaction because the acid and base components react to counterbalance each other making them chemically neutral. This type of reaction occurs between an acid and base to form a water and a salt. A neutralization reaction between a strong acid and a strong base react to form a neutral solution of pH 7.
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If 4.2 moles of 
is reacted with 3.5 moles of 
, how many grams of 
is produced?
If 4.2 moles of
is the limiting reactant because based on the equation, 
and 
react in a 1:1 mole ratio. However, there is 0.5 moles more of 
than 
therefore the amount of 
produced is limited by the amount of 
present in the reaction. Based on the equation for every 1 mole of 
reacted, 1 mole of 
was produced:
Therefore,
Therefore,
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Determine the acid dissociation constant for a 
monoprotic acid 
dissolved in the 
in which a pH meter for read pH 
for the solution.
Determine the acid dissociation constant for a
The acid dissociation constant expression for this reaction is:
Due to the dissociation of 
:
Using the following equation, we can calculate the 
concentration:
Plug the values obtained in to the acid dissociation constant expression:
The acid dissociation constant expression for this reaction is:
Due to the dissociation of
Using the following equation, we can calculate the
Plug the values obtained in to the acid dissociation constant expression:
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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.
In the reverse reaction of 
, the proton is acting as a(n) __________, and is thus a __________.
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.
In the reverse reaction of
In terms of the passage, the lone proton can be considered a proton donor and would, therefore, be a Brønsted-Lowry acid. This is not an answer choice.
The third acid-base definition is the Lewis definition, which states that acids are electron acceptors and bases are electron donors. The negative charge on the 
signifies that it is a Lewis base with available electrons to donate. The proton is accepting these electrons from 
, and is thus acting as a Lewis acid.
In terms of the passage, the lone proton can be considered a proton donor and would, therefore, be a Brønsted-Lowry acid. This is not an answer choice.
The third acid-base definition is the Lewis definition, which states that acids are electron acceptors and bases are electron donors. The negative charge on the
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What is the definition of a Brønsted-Lowry base?
What is the definition of a Brønsted-Lowry base?
A Brønsted-Lowry base is any compound that accepts protons in solution. Lewis acids and bases refer to the accepting or donating of an electron pair, respectively.
A Brønsted-Lowry base is any compound that accepts protons in solution. Lewis acids and bases refer to the accepting or donating of an electron pair, respectively.
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Which of the following molecules or ions have the greatest ability to act like a Lewis acid?
Which of the following molecules or ions have the greatest ability to act like a Lewis acid?
Lewis acids are electron pair acceptors. Molecules and ions that have a full octet cannot act a Lewis acid, therefore 
and 
are not lewis acids. 
is very stable and insoluble and cannot accept an electron pair. 
is a well known base and has extremely weak acidity. 
is a transition metal ion. Transition metal are known to be Lewis acids because of their positive charge which gives them the ability to accept electron pairs.
Lewis acids are electron pair acceptors. Molecules and ions that have a full octet cannot act a Lewis acid, therefore
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Which of the following is not a strong electrophile?
Which of the following is not a strong electrophile?
One of the easiest ways of determining if a molecule is an electrophile is by the presence of a positive charge. Electrophiles are in need of electrons, therefore they are electron deficient and can be attacked by nucleophiles (compounds that are electron rich). A nucleophile is a compound that provides a pair of electrons to form a new covalent bond. Nucleophiles are electron rich and one of the easiest types of nucleophiles to recognize are ones carrying a negative charge. 
is the only option given that contains a negative charge and therefore is not an electrophile.
One of the easiest ways of determining if a molecule is an electrophile is by the presence of a positive charge. Electrophiles are in need of electrons, therefore they are electron deficient and can be attacked by nucleophiles (compounds that are electron rich). A nucleophile is a compound that provides a pair of electrons to form a new covalent bond. Nucleophiles are electron rich and one of the easiest types of nucleophiles to recognize are ones carrying a negative charge.
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HCN dissociates based on the following reaction.
The Ka for hydrogen cyanide is 
.
of 
is added to 
of water. What is the pH of the resulting solution?
HCN dissociates based on the following reaction.
The Ka for hydrogen cyanide is
Since HCN is a weak acid, we must use the equilibrium equation.
Because the HCN dissociates in solution, we expect the concentrations of protons and cyanide ions to increase, while the concentration of HCN will decrease. After determining the molarity of the solution, we can set up the equation below, using X as the amount of moles that dissociate.
Because X is small, we can neglect its impact in the denominator.
Since X is the concentration of protons in the solution, we can calculate the pH by using the equation 
.
Since HCN is a weak acid, we must use the equilibrium equation.
Because the HCN dissociates in solution, we expect the concentrations of protons and cyanide ions to increase, while the concentration of HCN will decrease. After determining the molarity of the solution, we can set up the equation below, using X as the amount of moles that dissociate.
Because X is small, we can neglect its impact in the denominator.
Since X is the concentration of protons in the solution, we can calculate the pH by using the equation
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What is the molarity of a 
solution that has a pH of 
?
What is the molarity of a
The pH of the solution is 
, therefore the concentration would be 
. This solution is based on the equation, 
and because hydrochloric is a strong acid, it can be assumed to completely dissociate in solution.
The pH of the solution is
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