If the [A-]/[HA] ratio of a base/weak acid mixture changes from 10/1 to 1/10, how much does the pH change:

The pH changes by 2.0 if the [A-]/[HA] ratio of a base/weak acid mixture changes from 10/1 to 1/10.

Explanation:

To solve this problem we use the Henderson-Hasselbach equation:

pH = pKa + log [A⁻]/[HA]

Let's say we have a weak acid whose pKa is 7.0:

pH = 7.0 + log [A⁻]/[HA]

If the [A⁻]/[HA] ratio is 10/1, we're left with:

pH = 7.0 + log (10/1) pH = 7.0 + 1 pH = 8.0

Now if the ratio is 1/10:

pH = 7.0 + log (1/10) pH = 7.0 - 1 pH = 6.0

The difference in pH from one case to the other is (8.0-6.0) 2.0.

So the pH changes by 2.0 if the [A-]/[HA] ratio of a base/weak acid mixture changes from 10/1 to 1/10.

Keep in mind that no matter the value of pKa, the answer to this question will be the same.