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General Chemistry

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Acid-base Equilibrium

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Strong acids and bases ionize completely in aqueous solutions. Weak acids and bases ionize only partially in aqueous solutions. An acid-base reaction is shown below. A titration was done by adding a base to a known concentration and fixed volume of an acid. The reaction follows:

The dissociation of a weak acid can be represented as:

Chemists often use Henderson-Hasselbach equation to calculate the pH of solutions containing a weak acid of known pKa, provided that the other concentrations are known. Henderson-Hasselbach equation is:

This equation is also used to calculate the buffering capability of solutions. It is roughly estimated that a given buffer is most likely to be effective at pH levels between (pKa + 1) and (pKa – 1).

What are the base-to-acid ratios corresponding to the lower and upper limits of pH values which make the best buffer?

A 10 and 1
B 100 and 1
C 0.01 and 10
D 0.1 and 10
Ans. D In this question, we are asked to analyze the upper and lower limit ratios of base-to-acid at the best buffering pH. How can we answer this question? Well, there are some clues in the passage. According to the passage, the effective pH range of a buffer system is between pKa – 1 and pKa + 1. What other relevant information do we have in the passage? The passage also gives us an equation with which we can relate the pH and the pKa.

In order to have a pH value of pKa – 1, the second half of the equation with the log should be negative 1. For this the base-acid ratio should be 0.1; log 0.1 = –1. For the other criterion pKa + 1, the log value should be +1. For this, the ratio should be 10.

Acid-base Equilibrium Flashcard List

10 flashcards