Question

4. What conditions need to be present in order for the Henderson- Hasselbach equation to be used?

pH=pKa+log [A-]/[HA]

A- There needs to be equal amounts of an acid and a base.

B- Only a salt that contains non-spectator ions needs to be present.

C- There has to be conjugate pairs both initially present in the solution.

Answer 1

The correct answer is (C) There has to be conjugate pairs both initially present in the solution.

from the following equation we can see that the concentration of the Acid [HA] and the concentration of the conjugate base i.e. [A^-] is required, so we need the conjugate pairs to be present in the solution.

Henderson-Hasselbalch equation is also known as buffer equation. Let us consider a buffer of Acetic acid and Sodium acetate. From this we can observe that a salt that contains spectator ions is needed for this equation to be valid. Thus the second option is not true.

And about the first option, equal amount of acid and base requirement is totally illogical, since we basically calculate the pH of a buffer solution containing either weak acid or weak base, the total neutralisation does not occur. So equal amount of acid/base is not needed.

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