Question

A buffer that contains 0.34 M of an acid, HA and 0.37 M of its conjugate base A-, has a pH of 3.26. What is the pH after 0.032 mol of HCl are added to 0.71 L of the solution

Answer 1

Answer: pH ≈ 3.186

Explanation:

$p^{H} &=p^{k a}+\left[\frac{A^{-}}{H A}\right] \\3.26 &=p^{k a}+ \log \frac{0.37}{0.34} \\3.26 &=p^{k a}+\log\ 1.089 \\3.26 &=p^{k e}+0.037 \\p^{k a} &=3.26-0.1105 \\p^{k a} &=3.1495$

$0.34 \frac{\mathrm{mol}}{2} \times 0.71 \mathrm{~L}=0.2414 \ {\mathrm{mol} \rightarrow HA$

$0.37 \frac{\mathrm{mol}}{2} \times 0.71 \mathrm{~L}=0.2627 \ {\mathrm{mol} \rightarrow A^{-}$

$0.2414-0.032 \mathrm{mol}=0.2094 \mathrm{\ mol} \ HA$

$0.2627+0.032 \mathrm{mol}=0.2947 \mathrm{\ mol} \ A^{-}$

$\begin{aligned}&p H=3.1495+\log\frac{0.2627}{0.2414} \\&p H=3.1495+0.0367 \\&p H=3.186\end{aligned}$

Therefore, the pH after 0.032 mol of HCl are added to 0.71 L of the solution is approximately 3.186