Free Buffer Solutions MCQs with Answers

1. A buffer solution is one that

• A. has a pH of exactly 7
• B. neutralises all acids completely
• C. resists a change in pH when a small amount of acid or base is added
• D. conducts electricity better than water

Explanation: A buffer contains a reservoir of both a weak acid and its conjugate base, so added hydrogen ions are mopped up by the base and added hydroxide ions by the acid, and the pH barely moves. A buffer can be acidic or basic; a pH of 7 is not part of the definition. Blood is buffered near pH 7.4 mainly by the carbonic acid and hydrogen carbonate system.

2. Which mixture would act as an acidic buffer?

• A. Hydrochloric acid and sodium chloride
• B. Sodium hydroxide and sodium chloride
• C. Ammonia and ammonium chloride
• D. Acetic acid and sodium acetate

Explanation: An acidic buffer needs a weak acid together with the salt of that acid, and acetic acid with sodium acetate is the standard example. Ammonia with ammonium chloride is also a buffer but a basic one, since it is built from a weak base and its salt, which is why it is the closest distractor. Hydrochloric acid is strong and fully ionised, so it has no reservoir of un-ionised acid and cannot buffer.

3. The pH of a buffer solution made from a weak acid and its salt depends on

• A. the total volume of the solution
• B. the temperature only
• C. the presence of a catalyst
• D. the dissociation constant of the acid and the ratio of salt to acid concentration

Explanation: The Henderson Hasselbalch relationship gives pH as pKa plus the logarithm of the ratio of salt to acid, so it is the ratio that matters rather than the absolute amounts. Diluting the buffer changes both concentrations equally and so leaves the pH almost unaltered, which is one of its most useful features. Buffer capacity, however, does depend on how concentrated the components are.

4. Which of the following is basic buffer?

• A. NH4OH/NH4Cl
• B. NaOH/NaCl
• C. NaOH/HCl
• D. H2CO3/NaHCO3

Explanation: A basic buffer needs a weak base together with the salt of that base, and ammonium hydroxide with ammonium chloride is the standard example, holding the pH around 9. Carbonic acid with sodium hydrogen carbonate is also a buffer but an acidic one, built from a weak acid and its salt. Sodium hydroxide is a strong base and is fully ionised, so it has no reservoir of un-ionised base and cannot buffer.

5. What happens when H+ is added to ammonium hydroxide and ammonium chloride buffer?

• A. More ammonium hydroxide is formed
• B. Reaction will move reverse
• C. Reaction will move forward
• D. No effect on equilibrium

Explanation: The added hydrogen ions are mopped up by the weak base ammonium hydroxide, which reacts forward to give ammonium ions and water, so the pH barely shifts. This is exactly what buffering means: the reservoir of base neutralises added acid. If alkali were added instead, the ammonium ions from the salt would supply protons to neutralise it.