Question

A beaker with 2.00×10^2 mL of an acetic acid buffer with a pH of 5.000 is sitting on a benchtop. The total molarity of acid and conjugate base in this buffer is 0.100 M. A student adds 8.40 mL of a 0.300 M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.740. Express your answer numerically to two decimal places. Use a minus (−) sign if the pH has decreased.

Answer 1

Let 0.100 M buffer contains x M acetate and 0.100-x M acetic acid.

Thus, 0.100 M buffer contains 0.064535 M acetate and M acetic acid.

mL of the buffer contains moles of acetate and moles of acetic acid.

Now, 8.40 mL of a 0.300 M HCl solution is added.

The number of moles of HCl added

0.00252 moles of HCl will react with 0.00252 moles of sodium acetate to form 0.00252 moles of acetic acid.

Total number of moles of acetic acid

Number of moles of sodium acetate remaining

The change in pH

Use a minus (−) sign if the pH has decreased.

The change in pH