In order to prepare a buffer solution of pH 5.74, sodium acetate is added to acetic acid. If the concentration of acetic acid in the buffer is 1.0 M, the concentration of sodium acetate in the buffer is ___________ M. (Round off to the Nearest Integer). [Given : pKa (acetic acid) = 4.74]

Solution

Chemistry ma’am · 3 steps

pH is exactly 1 unit above pKa, so the salt must be ten times the acid. That is the whole question.

1. Acidic buffer. Acetic acid with sodium acetate is an acidic buffer. Its pH follows the Henderson–Hasselbalch equation.

   pH = pKₐ + log[CH₃COO⁻]/[CH₃COOH]

2. Put in numbers. 5.74 = 4.74 + log[salt]/1.0, so log [salt] = 1. That gives [salt] = 10¹ = 10 M.
3. So. The concentration of sodium acetate in the buffer is 10 M.

The common slip: 0.1. Flipping the ratio to acid over salt gives log [salt] = -1 and a salt concentration of 0.1 M, not 10 M.

The idea to keep. In a buffer, every 1 unit that pH sits above pKa means the salt-to-acid ratio goes up ten times. Below pKa, the acid wins by the same factor.

Answer from the NTA final answer key. Solution by PrepWiser.