Solution A is a 1.00 L buffer solution that is 1.188 M in acetic acid and 1.188 M in sodium acetate. Acetic acid has a pKa of 4.74. What is the pH change of this solution upon addition of 0.1 mol of HCl? Enter your answer numerically to four decimal places. It will be a negative number.

pH change is -0.07

Explanation:

Using H-H equation for acetic acid:

pH = pKa + log [Acetate salt] / [Acetic acid]

Replacing:

pH = 4.74 + log[1.188M] / [1.188M]

pH = 4.74

The HCl reacts with sodium acetate producing acetic acid, thus:

HCl + CH₃COONa → CH₃COOH + NaCl

That means the final moles of sodium acetate are initial moles - moles of HCl and moles of acetic acid are initial moles + moles of HCl.

As the volume of the buffer is 1.0L, initial moles of both substances are 1.188moles. After reaction, the moles are:

sodium acetate: 1.188mol - 0.1mol = 1.088mol

Acetic acid: 1.188mol + 0.1mol = 1.288mol

Using again H-H equation:

pH = 4.74 + log[1.088M] / [1.288M]

pH = 4.67

pH change is: 4.67 - 4.74 = -0.07

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