ONE-EIGHTH STAGE 2026
ROUND 4 - TRUE OR FALSE
Aburi Girl’s SHS - 38 points
St. Joseph’s Seminary SHS - 20 points
Fafraha Community SHS - 12 points
St. John's Grammar School - 40
Mpraeso SHS - 23
Tema Secondary School – 13
St. Augustine’s College
Anlo SHS
Kumasi High School
PREAMBLE
A buffer solution is prepared from a weak acid A and its salt B (conjugate base).
Recall the Henderson-Hasselbalch equation: pH = pKa + log([B]/[A]).
1. If more salt B is added to the solution, the pH of the buffer rises.
ANSWER: TRUE
EXPLANATION:
Adding more B increases the ratio [B]/[A], increasing log([B]/[A]) and therefore raising the pH.
2. If the buffer is diluted tenfold with pure water, its pH falls by one unit.
ANSWER: FALSE
EXPLANATION:
Diluting a buffer decreases [A] and [B] by the same factor, so the ratio [B]/[A] — and therefore the pH — stays essentially unchanged.
Resisting pH change on dilution is a defining property of a buffer.
3. If the ratio of the concentration of B to the concentration of A equals 1, then the pH of the buffer equals the pKa of A.
ANSWER: TRUE
EXPLANATION:
pH = pKa + log([B]/[A]) = pKa + log(1) = pKa + 0 = pKa.
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PRACTICE QUESTIONS
PREAMBLE 1
A buffer solution contains ethanoic acid and sodium ethanoate in equal concentrations.
1. The pH of the buffer equals the pKa of ethanoic acid.
ANSWER: TRUE
EXPLANATION:
With [salt] = [acid], $\log\dfrac{[\text{salt}]}{[\text{acid}]} = \log1 = 0$, so pH = pKa.
2. Adding a little more ethanoic acid raises the pH of the buffer.
ANSWER: FALSE
EXPLANATION:
More acid lowers the ratio [salt]/[acid], so the pH falls slightly.
3. A small amount of added NaOH is removed by reaction with ethanoic acid.
ANSWER: TRUE
EXPLANATION:
$CH_3COOH + OH^- \rightarrow CH_3COO^- + H_2O$, so the pH changes very little.
PREAMBLE 2
A buffer solution contains ammonia and ammonium chloride in equal concentrations.
1. A little added hydrochloric acid is removed by reaction with ammonia.
ANSWER: TRUE
EXPLANATION:
$NH_3 + H^+ \rightarrow NH_4^+$, so the pH changes very little.
2. The pH of this buffer is below 7.
ANSWER: FALSE
EXPLANATION:
The pKa of $NH_4^+$ is about 9.3, so with equal concentrations the pH is about 9.3.
3. Diluting the buffer with a little water hardly changes its pH.
ANSWER: TRUE
EXPLANATION:
Both concentrations fall by the same factor, so their ratio and the pH stay almost the same.