An acetic acid buffer containing 0.50 M CH₃COOH and 0.50 M CH₃COONa has a pH of 4.74. What will the pH be after 0.0020 mol of HCl has been added to 100.0 mL of the buffer?

Question

Quizplus · Accepted Answer

First, we can use the Henderson-Hasselbalch equation to calculate the initial pH of the buffer:

pH = pKa + log([CH3COO-]/[CH3COOH])

We can look up the pKa of acetic acid to be 4.76. Plugging in the given concentrations of the buffer components, we get:

pH = 4.76 + log([0.50]/[0.50]) = 4.76

So the initial pH of the buffer is 4.76.

Next, we need to consider the effect of adding HCl. HCl is a strong acid, so it will react completely with the acetate buffer system to form more H+ and acetate ion (CH3COO-):

HCl + CH3COO- --> CH3COOH + Cl-

Since the initial concentrations of the buffer components are equal, we can assume that half of the added HCl will react with CH3COO- to form more CH3COOH, and the other half will remain as HCl:

0.0020 mol HCl * (1/2) = 0.0010 mol CH3COO- reacts

This will consume some of the CH3COO- in the buffer and shift the equilibrium to the left, decreasing the concentration of CH3COO- and increasing the concentration of CH3COOH. To calculate the new equilibrium concentrations, we can use an ICE table:

I:    CH3COOH   +   CH3COO-   -->   H+   +   CH3COO-
C:    0.50      -   0.0010     +    x         +  0.0010
E:    0.50 -x        0.499       x          0.0010

Next, we can use the new concentrations of CH3COOH and CH3COO- in the Henderson-Hasselbalch equation to find the new pH:

pH = pKa + log([CH3COO-]/[CH3COOH])
pH = 4.76 + log([0.499]/[0.50 - 0.0010])
pH = 4.71

Therefore, the final pH after adding 0.0020 mol of HCl is 4.71, which corresponds to answer choice B.

An Acetic Acid Buffer Containing 0