Q: A river water sample contains $[\mathrm{HCO_3^-}] = 1.0 \times 10^{-4}\ \mathrm{M}$ and $[\mathrm{H_2CO_3}] = 1.4 \times 10^{-5}\ \mathrm{M}$. Given $pK_a = 6.37$, what is the pH of the sample?

$pH = 6.37 + \log \left( \frac{1.0 \times 10^{-4}}{1.4 \times 10^{-5}} \right) = 7.22$

Question 1

Which of the following best describes the common ion effect in aqueous equilibrium systems?

Accepted Answer

It is the shift in position of an equilibrium caused by the addition of an ion already present in the reaction.

Question 2

Given the equilibrium $\mathrm{H_2CO_3(aq) + H_2O(\ell) ightleftharpoons HCO_3^-(aq) + H_3O^+(aq)}$ with $K_{a1} = 4.3 	imes 10^{-7}$, what will happen to the ionization of $\mathrm{H_2CO_3}$ if $\mathrm{HCO_3^-}$ is added to the solution?

Accepted Answer

Ionization of $\mathrm{H_2CO_3}$ will decrease.

Question 3

A buffer solution is prepared using acetic acid ($\mathrm{CH_3COOH}$) and sodium acetate. Which equation correctly represents the Henderson-Hasselbalch equation for this buffer?

Accepted Answer

$pH = pK_a + \log \left( \frac{[\mathrm{CH_3COO^-}]}{[\mathrm{CH_3COOH}]} \right)$

Question 4

A river water sample contains $[\mathrm{HCO_3^-}] = 1.0 	imes 10^{-4}\ \mathrm{M}$ and $[\mathrm{H_2CO_3}] = 1.4 	imes 10^{-5}\ \mathrm{M}$. Given $pK_a = 6.37$, what is the pH of the sample?

Accepted Answer

$pH = 6.37 + \log \left( \frac{1.0 	imes 10^{-4}}{1.4 	imes 10^{-5}} ight) = 7.22$

Question 5

Which statement about buffer solutions is correct?

Accepted Answer

A buffer resists changes in pH when small amounts of acid or base are added.

Question 6

If you want to prepare 250.0 mL of a buffer solution with pH 4.70 using 0.16 M acetic acid ($K_a = 1.76 	imes 10^{-5}$), how many moles of sodium acetate are needed?

Accepted Answer

0.0358 moles

Question 7

During the titration of a weak acid with a strong base, which of the following is true at the equivalence point?

Accepted Answer

The solution contains only the conjugate base of the weak acid.

Question 8

Which indicator would be most suitable for a titration between a strong acid and a strong base?

Accepted Answer

An indicator with a color change near pH 7

Question 9

A solution contains $\mathrm{Mg(OH)_2(s)}$ in equilibrium with $\mathrm{Mg^{2+}(aq)}$ and $\mathrm{OH^-(aq)}$. If more $\mathrm{OH^-}$ is added, what happens to the solubility of $\mathrm{Mg(OH)_2}$?

Accepted Answer

Solubility decreases due to the common ion effect.

Question 10

Which of the following is a correct application of the Lewis acid-base theory?

Accepted Answer

$\mathrm{NH_3}$ acts as a Lewis base by donating a lone pair to $\mathrm{H^+}$.

Question 11

Which of the following will increase the solubility of a salt containing a basic anion, such as $\mathrm{CaF_2}$?

Accepted Answer

Adding a strong acid to the solution

Question 12

A solution contains $[\mathrm{Pb^{2+}}] = 0.0921\ \mathrm{M}$ and $[\mathrm{Cl^-}] = 0.0106\ \mathrm{M}$. If $K_{sp}$ for $\mathrm{PbCl_2}$ is $1.7 	imes 10^{-5}$, will a precipitate form?

Accepted Answer

No, because $Q = (0.0921)(0.0106)^2 = 1.03 	imes 10^{-5} < K_{sp}$

Question 13

Which of the following best explains why the pH at the equivalence point in a weak acid-strong base titration is greater than 7?

Accepted Answer

The conjugate base of the weak acid hydrolyzes to produce $\mathrm{OH^-}$ ions.

Question 14

Which of the following is true about buffer capacity?

Accepted Answer

Buffer capacity increases with higher concentrations of buffer components.

Question 15

Which of the following is a correct statement about the pH range of a buffer?

Accepted Answer

The effective pH range of a buffer is $pK_a \pm 1$.

Question 16

During the titration of acetic acid ($\mathrm{CH_3COOH}$) with NaOH, after adding 5.0 mL of 0.100 M NaOH to 20.0 mL of 0.100 M acetic acid, what is the ratio of moles of acetate to acetic acid remaining?

Accepted Answer

0.50 mmol acetate to 1.50 mmol acetic acid

Question 17

Which of the following is a correct definition of a complex ion?

Accepted Answer

An ionic species consisting of a metal ion bonded to one or more Lewis bases

Question 18

Which of the following will NOT affect the solubility of a slightly soluble salt?

Accepted Answer

Adding a non-reactive gas to the solution

Chapter 16: Additional Aqueous Chemistry and the Oceans – Equilibria, Buffers, and Acid-Base Calculations

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