A buffer solution is composed of 7.50 mol of acid and 4.75 mol of the conjugate base. If the pKa of the acid is 4.90 , what is the pH of the buffer?

The correct answer is: pH= 4.70

Explanation:

We use the Henderson-Hasselbach equation in order to calculate the pH of a buffer solution:

Given:

pKa= 4.90

[conjugate base]= 4.75 mol

[acid]= 7.50 mol

We calculate pH as follows:

pH = 4.90 + log (4.75 mol/7.50 mol) = 4.90 + (-0.20) = 4.70