Question 1

Which of the following best describes the common ion effect in an aqueous solution?

Accepted Answer

The addition of an ion already present in equilibrium shifts the equilibrium to favor the formation of more reactants.

Question 2

Given the equilibrium $$\mathrm{CH_3COOH(aq) ightleftharpoons H^+(aq) + CH_3COO^-(aq)}$$ with $$K = 1.8 	imes 10$$^{-5}$$, what is the initial concentration of $$\mathrm{CH_3COO^-}$$ in a solution prepared by mixing 0.30 mol of acetic acid and 0.30 mol of sodium acetate in 1.0 L of water?

Accepted Answer

$$0.30\ \mathrm{M}$$

Question 3

Which statement correctly predicts the effect of adding sodium acetate to a solution of acetic acid, according to Le Chatelier's Principle?

Accepted Answer

The equilibrium will shift to produce more $$\mathrm{CH_3COOH}.$$

Question 4

A solution contains 0.050 M formic acid ($$\mathrm{HCOOH}) $$and 0.10 M nitric acid ($$\mathrm{$$HNO_3$$}). $$Given $$K = 1.8 	imes 10$$^{-4}$$ for formic acid, which ion acts as the common ion in this system?

Accepted Answer

$$\mathrm{H^+}$$

Question 5

In the equilibrium $$\mathrm{CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)}$$, what is the effect on $$[\mathrm{H^+}]$$ when sodium acetate is added to the solution?

Accepted Answer

$$[\mathrm{H^+}]$$ decreases

Question 6

Which equation represents the Law of Mass Action for the equilibrium $$\mathrm{CH_3COOH(aq) \rightleftharpoons H^+(aq) + CH_3COO^-(aq)}$$?

Accepted Answer

$$K = \dfrac{[\mathrm{H^+}][\mathrm{CH_3COO^-}]}{[\mathrm{CH_3COOH}]}$$

Question 7

A student prepares a solution by mixing 0.30 M acetic acid and 0.30 M sodium acetate. Which of the following best describes the change in $$[\mathrm{H^+}]$$ compared to a solution with only 0.30 M acetic acid?

Accepted Answer

$$[\mathrm{H^+}] is $$lower in the mixed solution

Question 8

Which of the following is a correct qualitative prediction about a solution containing both acetic acid and sodium acetate?

Accepted Answer

The solution will act as a buffer, resisting changes in pH.

Question 9

If the equilibrium constant for formic acid is $$K = 1.8 	imes 10$$^{-4}$$, which of the following expressions correctly represents the equilibrium constant?

Accepted Answer

$$K = \dfrac{[\mathrm{H^+}][\mathrm{HCOO^-}]}{[\mathrm{HCOOH}]}$$

Question 10

A solution is made by adding 0.30 mol of acetic acid and 0.30 mol of sodium acetate to 1.0 L of water. Which calculation would you use to determine the equilibrium concentration of $$\mathrm{H^+}$$?

Accepted Answer

Question 11

Which step is necessary when using an ICE table to solve for equilibrium concentrations in a system affected by the common ion effect?

Accepted Answer

Include the initial concentration of the common ion from all sources.

Question 12

A solution contains both formic acid and nitric acid. Why does the presence of nitric acid affect the equilibrium of formic acid?

Accepted Answer

Nitric acid provides additional $$\mathrm{H^+}$$ ions, shifting the equilibrium of formic acid toward the reactant side.

Question 13

Which of the following best explains why a buffer solution is able to resist changes in pH?

Accepted Answer

The buffer contains a weak acid and its conjugate base, allowing it to neutralize added acids or bases.

Question 14

If the equilibrium constant $$K$$ for acetic acid is $$1.8 	imes 10$$^{-5}$$, what is the effect of increasing $$[\mathrm{CH_3COO^-}]$$ on the equilibrium concentration of $$[\mathrm{H^+}]$$?

Accepted Answer

$$[\mathrm{H^+}]$$ decreases

Question 15

A medical solution is buffered with acetic acid and sodium acetate. If a small amount of strong acid is added, what happens to the pH?

Accepted Answer

The pH decreases slightly, but the buffer resists a large change.

Question 16

Which of the following is a quantitative prediction about a solution containing a common ion?

Accepted Answer

The equilibrium concentration of the weak acid decreases.

Question 17

In a solution containing both acetic acid and sodium acetate, which equation would you use to calculate the pH?

Accepted Answer

$$\mathrm{pH} = \mathrm{p}K_a + \log\left(\dfrac{[\mathrm{CH_3COO^-}]}{[\mathrm{CH_3COOH}]}\right)$$

Question 18

A student uses an ICE table to solve for equilibrium concentrations in a solution with a common ion. Which value should be used for the initial concentration of the common ion?

Accepted Answer

The sum of the concentrations from all sources.

Question 19

Which scenario best illustrates the application of the common ion effect in a real-world context?

Accepted Answer

Adding sodium acetate to a solution of acetic acid to create a buffer for medical use.

The Common Ion Effect and Equilibrium Calculations

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