FE Environmental Environmental Chemistry Practice Problems

The current FE Environmental specification assigns 7-11 of the 110 exam questions to Environmental Chemistry. Form A contains 8 original problems in this area. The sample below is published in full, with the same worked-solution format used throughout the book.

A laboratory acetate buffer contains 0.050 mol/L acetic acid and 0.030 mol/L sodium acetate, with $p K_{a} = 4.76$ . Strong acid is then added to a concentration of 0.010 mol/L with negligible volume change. The pH after the addition is most nearly:

(A)4.28
(B)4.36
(C)4.54
(D)5.24

Show worked solution

Answer: (A)

Converting a third of the acetate to acetic acid triples the acid-to-base ratio and pulls the pH down to about 4.28.

Added protons convert acetate to acetic acid mole for mole, so both members of the pair change before any equilibrium relation is written.

[HA] = 0.050 + 0.010 = 0.060 mol/L

[A^{-}] = 0.030 - 0.010 = 0.020 mol/L

Henderson-Hasselbalch governs because both conjugate members survive the addition in large excess, which is precisely the buffer capacity being tested.

pH = p K_{a} + lo g \frac{[ A ^{-} ]}{[ HA ]} = 4.76 + lo g \frac{0.020}{0.060} = 4.28

FE Reference Handbook — Chemistry: Acid-Base Equilibria (Henderson-Hasselbalch Equation)

Why the other choices appear

(B)Reduces the acetate to 0.020 mol/L but leaves the acid at 0.050 mol/L, giving 4.76 + log(0.4) = 4.36.
(C)Reports the buffer pH before the acid addition, 4.76 + log(0.030/0.050) = 4.54.
(D)Inverts the ratio in the logarithm, giving 4.76 + log(0.060/0.020) = 5.24.