AP Chemistry 8.3: Weak Acid and Base Equilibrium

Practice AP Chemistry questions on calculating pH, Ka, Kb, pKa, pKb, equilibrium concentrations, and percent ionization for weak acids and bases.

Exam points

Explain how reaction rates depend on concentration, activation energy and molecular collision pathways.
Apply rate laws, catalysts and concentration data to predict reaction speed and reaction order.

[Maximum number: 2]

Answer the following questions about the glycolate ion, $\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{3}{ }^{-}$ , which acts as a base in

aqueous solution. A Lewis diagram for the ion is provided.

Figure for Question 8.3.A—Explain the relationship among pH, pOH, and concentrations of all species in a solution of a monoprotic weak… question 1 — AP Chemistry

(a)

At $25^{\circ} \mathrm{C}$ , a 2.5 M solution of glycolate is found to have $\left[\mathrm{OH}^{-}\right]=1.3 \times 10^{-5} M$ .

[ 2 ]

(i)

Calculate the value of $K_{b}$ for the glycolate ion.

[ 1 ]

B (i) For the correct calculated value:

Point 02

$K_{b}=\frac{\left[\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{3}\right]\left[\mathrm{OH}^{-}\right]}{\left[\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{3}^{-}\right]}=\frac{\left(1.3 \times 10^{-5}\right)\left(1.3 \times 10^{-5}\right)}{\left(2.5-1.3 \times 10^{-5}\right)} \approx \frac{\left(1.3 \times 10^{-5}\right)^{2}}{(2.5)}=6.8 \times 10^{-11}$

(ii)

Using your answer to part B (i), calculate the value of $K_{a}$ for glycolic acid at 25°C.

[ 1 ]

(ii) For the correct calculated value, consistent with part B (i):

Point 03

$K_{a}=\frac{K_{w}}{K_{b}}=\frac{1.0 \times 10^{-14}}{6.8 \times 10^{-11}}=1.5 \times 10^{-4}$

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8.3.A—Explain the relationship among pH, pOH, and concentrations of all species in a solution of a monoprotic weak… question 1

Question (a)

Question (i)

Question (ii)