Acid-Base Reactions

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AP Chemistry › Acid-Base Reactions

Questions 1 - 10
1

A buffer using acetic acid (pKa=4.76) is titrated with NaOH. What is the pH at half the equivalence point?

2.38

4.76

7.00

9.52

12.36

Explanation

The pH at half the equivalence point is equal to the pKa of the acid.

2

Determine the pH of a solution that is .

Explanation

Since is a strong acid, the concentration of is equal to the concentration of the acid itself.

Thus, .

Recall how to find the pH of a solution:

Plug in the given hydronium ion concentration to find the pH of the given solution.

Remember to maintain the correct number of significant figures.

3

Put the following acids in order of their INCREASING acid strength: HI, HCl, HBr, HF.

HI, HCl, HBr, HF

HF, HCl, HBr, HI

HI, HBr, HCl, HF

HF, HBR, HI, HCl

HCl, HBr, HI, HF

Explanation

Larger halogen size leads to greater acidity because of weaker H-X interactions.

4

If you have a solution that consists of a monoprotic acid (HA), with a pKa of 4.1 and at a pH of 5.8, what is the predominant species present?

HA

A­-

H2A+

H3O+

Equal amounts of acid and conjugate base are present.

Explanation

Since pH > pKA, the deprotonated form of the acid is predominant.

5

Where does the flattest region of a titration curve of the titration of a weak acid with a strong base occur?

At the pKa of the acid

At the pKb of the base

At a pH greater than 7

At a pH of 7

Explanation

In this question, titration curve would graph the pH of acid solution versus the amount of base added. Since the base is strong and the acid is weak, we can conclude that the pH will be slightly greater than 7 at the equivalence point. The equivalence point is found in the steepest region of the curve.

The half-equivalence point is the flattest region of the titration curve and is most resistant to changes in pH. This corresponds to the pKa of the acid. Within this region, adding base (changing the x-value) results in very little deviation in the pH (the y-value). This region is also the buffer region for the given acid.

6

Which of the following will increase the pH of an buffer solution?

I. Removing carbonic acid

II. Adding sodium bicarbonate

Both I and II

I only

II only

Neither of these options

Explanation

To answer this question we need to look at the reaction below:

An increase in the pH will result in a decrease in the concentration of hydrogen ions (). Using Le Chatelier’s principle we can find out which answer choices will decrease .

Removing carbonic acid will decrease the concentration of . To maintain equilibrium, the reaction will shift to the left and make more reactants from products; therefore, there will be a decrease in the and an increase in pH.

Recall that salts like sodium bicarbonate, or , will dissociate in water and form ions. Sodium bicarbonate will form sodium () and bicarbonate () ions. This side reaction will result in an increase in the bicarbonate ion concentration. Le Chatelier’s principle will shift the equilibrium of the given reaction to the left and, therefore, decrease the . Adding sodium bicarbonate will increase the pH.

7

Which of the following is what determines the strength of an acid?

The Ka

The Kb

Its physical state

How many bonds the central atom makes

Electronegativity values

Explanation

The Ka is the acid dissociation constant, and thus it is what determines how strong the acid is. Stronger acids dissociate to a greater extent and produce lower pH values.

8

Where does the flattest region of a titration curve of the titration of a weak acid with a strong base occur?

At the pKa of the acid

At the pKb of the base

At a pH greater than 7

At a pH of 7

Explanation

In this question, titration curve would graph the pH of acid solution versus the amount of base added. Since the base is strong and the acid is weak, we can conclude that the pH will be slightly greater than 7 at the equivalence point. The equivalence point is found in the steepest region of the curve.

The half-equivalence point is the flattest region of the titration curve and is most resistant to changes in pH. This corresponds to the pKa of the acid. Within this region, adding base (changing the x-value) results in very little deviation in the pH (the y-value). This region is also the buffer region for the given acid.

9

Put the following acids in order of their INCREASING acid strength: HI, HCl, HBr, HF.

HI, HCl, HBr, HF

HF, HCl, HBr, HI

HI, HBr, HCl, HF

HF, HBR, HI, HCl

HCl, HBr, HI, HF

Explanation

Larger halogen size leads to greater acidity because of weaker H-X interactions.

10

Which of the following will increase the pH of an buffer solution?

I. Removing carbonic acid

II. Adding sodium bicarbonate

Both I and II

I only

II only

Neither of these options

Explanation

To answer this question we need to look at the reaction below:

An increase in the pH will result in a decrease in the concentration of hydrogen ions (). Using Le Chatelier’s principle we can find out which answer choices will decrease .

Removing carbonic acid will decrease the concentration of . To maintain equilibrium, the reaction will shift to the left and make more reactants from products; therefore, there will be a decrease in the and an increase in pH.

Recall that salts like sodium bicarbonate, or , will dissociate in water and form ions. Sodium bicarbonate will form sodium () and bicarbonate () ions. This side reaction will result in an increase in the bicarbonate ion concentration. Le Chatelier’s principle will shift the equilibrium of the given reaction to the left and, therefore, decrease the . Adding sodium bicarbonate will increase the pH.

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