Question

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

Answer 1

`pH = 5.05`

Step-by-step 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:

`(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:

`(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 = ([H^+][A^-])/([HA])`

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

`K_a = (x^2)/(0.125786-x)`

`[H^+] = 8.83*10^(-6)`

`pH = -log[H^(+) ]`

`pH = -log(8.83 * 10^(-6) )`

`pH = 5.05`