AP Chemistry Unit 8.3: Weak Acid and Base Equilibrium

Practice AP Chemistry Unit 8.3 questions on pH, Ka, Kb, pKa, pKb, equilibrium concentrations, percent ionization, and dilution.

Exam points

Use molecular geometry and bonding to predict polarity, solubility, phase behavior, and properties.
Interpret particulate diagrams and structural data to explain bonding and interactions determine chemical behavior.

Covered syllabus points

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

[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 Weak Acid and Base Equilibria 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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Covered syllabus points

8.3 Weak Acid and Base Equilibria question 1

Question (a)

Question (i)

Question (ii)