An acetate buffer is 0.10 M in acetic acid and 0.10 M in sodium acetate, with Ka = 1.8 x 10^-5. What is its pH?

1. Set out the equilibrium with acetic acid at 0.10 M and acetate also at 0.10 M. 2. Solving for the hydronium concentration gives x = 1.8 x 10^-5 mol per litre. 3. That is the acid constant itself, because the two buffer components are equal. 4. Take the negative logarithm: minus log of 1.8 x 10^-5 gives 4.74. 5. So the buffer sits at pH 4.74, which is the pKa of acetic acid. 6. The value 2.87 belongs to 0.100 M acetic acid with no acetate added. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.6 Buffers_