Acid-Base Chemistry

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AP Chemistry › Acid-Base Chemistry

Questions 1 - 10
1

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.

2

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.

3

You are presented with a solution that has a pOH of 2.13. What is the pH of this solution?

Explanation

pH and pOH are the log concentrations of protons and hydroxide ions, respectively.

The sum of pH and pOH is always 14. This is because the product of proton concentration and hydroxide concentration must always equal the equilibrium constant for the ionization of water, which is equal to .

In this question, we know that the pOH is equal to 2.13, allowing us to solve for the pH.

4

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.

5

Calculate the pH of the following solution at :

Explanation

Use the the dissociation constant for water to calculate the concentration of hydrogen ions and then convert to the pH scale.

Rearrange the dissociation constant to solve for the hydrogen ion concentration:

Plug in the given concentrations to find the actual hydrogen ion concentration:

Convert the hydrogen ion concentration to pH scale:

6

How does the Brønsted-Lowry theory of acids and bases describe acids?

Brønsted-Lowry acids are proton donors when dissolved in water

Brønsted-Lowry acids are proton acceptors when dissolved in water

Brønsted-Lowry acids are any compound that dissociates in water to yield a proton

Brønsted-Lowry acids are electron acceptors when dissolved in water

Brønsted-Lowry acids are acids that dissociate 100% in a solution

Explanation

There are three primary classifications of acids and bases.

Arrhenius acids yield protons when dissolved in solution, while Arrhenius bases yield hydroxide ions.

Brønsted-Lowry acids are protone donors, while Brønsted-Lowry bases are proton acceptors.

Lewis acids are electron acceptors, while Lewis bases are electron donors.

An acid that dissociates completely in solution is considered a strong acid due to its high Ka value.

7

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.

8

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.

9

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.

10

What is the Bronsted-Lowry definition of acids?

Acids are hydrogen donors

Acids are hydrogen acceptors

Acids are electron pair acceptors

Acids are electron pair donors

Acids are amphoteric

Explanation

According to the Bronsted-Lowry definition, acids are hydrogen donors.

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