A buffer solution contains 0.0200 M acetic acid and 0.0200 M sodium acetate.What is the pH after 2.0 mmol of HCl is added to 1.00 L of this buffer? pK_a = 4.75 for acetic acid.

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When 2.0 mmol of HCl is added to the buffer, it reacts with the sodium acetate, converting it to acetic acid. This decreases the concentration of acetate ions and increases the concentration of acetic acid. The change in concentrations can be calculated as follows: the initial moles of acetic acid and acetate are both 0.0200 mol (since the volume is 1.00 L and their concentrations are 0.0200 M). After the addition of 2.0 mmol (or 0.0020 mol) of HCl, the moles of acetate decrease by 0.0020 mol, and the moles of acetic acid increase by 0.0020 mol. Therefore, the new concentrations are 0.0220 M for acetic acid and 0.0180 M for acetate.The pH of the buffer solution can be calculated using the Henderson-Hasselbalch equation:$$ \text{pH} = \text{p}K_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right) $$$$ \text{pH} = 4.75 + \log\left(\frac{0.0180}{0.0220}\right) $$$$ \text{pH} = 4.75 + \log(0.8182) $$$$ \text{pH} = 4.75 - 0.087 $$$$ \text{pH} = 4.663 $$Rounding to two decimal places, the pH is approximately 4.66.

A Buffer Solution Contains 0