Help with Buffers

Help Questions

AP Chemistry › Help with Buffers

Questions 1 - 10
1

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

None of these

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.

2

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

None of these

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.

3

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.

4

Which of the following combinations cannot be used to produce a buffer solution?

and

and

and

and

Explanation

Buffer solutions can be made via two methods. The first method involves adding equal amounts of a weak acid and a salt of its weak conjugate base (or vice versa). The second methods involves adding a weak acid and a half equivalent of a strong base (or vice versa).

is a weak acid and is a salt of its weak conjugate base; therefore, this can form a buffer.

is a weak base and is a salt of its weak conjugate acid; this can also form a buffer. Note that this is the converse of the first method (weak base with salt of weak acid), but it can still form a buffer solution.

is a strong acid and is a weak base; therefore, adding and a half equivalent of will create a buffer solution. This is the converse of the second method (adding a weak base to a half equivalent of strong acid).

and are both strong reagents (acid and base, respectively); therefore, they cannot form a buffer solution.

5

Which of the following would be most useful as a buffer?

A solution of ammonia and ammonium chloride

A solution of sodium chloride and sodium hydroxide

Potassium hydroxide

Water

A solution of carbonic acid and sodium chloride

Explanation

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.

6

A buffer solution is a solution that __________.

consists of a mixture of a weak acid and its conjugate base, which resists change in pH

allows for a solution to easily change pH value without requiring great addition of strong acid or strong base

is slightly acidic and is used in laboratory settings to prevent chemical reactions from taking place

is slightly basic and is used in laboratory settings to prevent chemical reactions from taking place

None of these

Explanation

A buffer solution is used to keep the pH of the solution within a relatively narrow range. It must resist change in pH upon addition of acids or bases. A weak acid is used to lower the pH in response to addition of base or removal of acid, and its conjugate base is used to raise the pH in response to addition of acid or removal of base.

7

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 ?

Explanation

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.

8

Which of the following solutions has the greatest buffering capacity?

1M Acetic Acid

2M Formic Acid

4M Nitric Acid

3M Rubidium Hydroxide

Explanation

Nitric Acid is a strong acid and can't buffer. Rubidium Hydroxide is a strong base and thus can't buffer. Of the remaining, both are weak acids, but the one with a greater concentration has a greater buffering capacity.

9

To create a buffer solution, you can use a weak acid and .

its conjugate base

another weak acid

a strong base

its conjugate acid

Explanation

The definition of a buffer solution is that it contains a weak acid and its conjugate base, or a weak base and its conjugate acid. Since we are starting with a weak acid in this case, we need its conjugate base.

10

Which of the following would best buffer a solution from a pH of 4 to 6?

Acetic acid (pKa = 4.7)

Carbonic acid (pKa = 6.3)

Formic acid (pKa = 3.7)

Hydrocyanic acid (pKa = 9.2)

Uric acid (pKa = 3.9)

Explanation

A weak acid/base best buffers about 1 pH point above and below its pKa. The pKA closest to the middle of 4 and 6 (so want as close to 5) is acetic acid at 4.7.

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