Question

What is the pH of a 900 mL solution containing 3.40 grams of hydrocyanic acid

Answer 1

Answer:

$pH = 5.05$

Explanation:

pH is derived from the concentration of hydronium ions in a solution. Hydrocyanic acid is HCN.

First, we shall figure out the moles of HCN:

$\frac{3.4g}{27.03g/mol} = 0.125786$

If HCN was a strong acid:

HCN has a 1:1 ratio of H+ ions, the moles of H+ is also the same.

To find the molarity, we now divide by Liters. This gets us:

$\frac{0.125786 moles}{0.9L} = 0.139762 M$

Finally, we plug it into the definition of pH:

$pH = -log[H^{+} ]$

$pH = -log(0.139762)$

$pH = 0.855$

However, since HCN is a weak acid, it only partially dissociates. The $K_a$ of HCN is $6.2 * 10^{-10}$.

$K_a = \frac{[H^+][A^-]}{[HA]}$

We can use an ice table to determine that when x = H+,

$K_a = \frac{x^2}{0.125786-x}$

$[H^+] = 8.83*10^{-6}$

$pH = -log[H^{+} ]$

$pH = -log(8.83 * 10^{-6} )$

$pH = 5.05$