# What is the pH of 2 M aluminium nitrate ?

May 17, 2017

$\textsf{p H = 2.62}$

#### Explanation:

The ion undedrgoes hydrolysis:

$\textsf{{\left[A l {\left({H}_{2} O\right)}_{6}\right]}^{3 +} r i g h t \le f t h a r p \infty n s {\left[A l {\left({H}_{2} O\right)}_{5} O H\right]}^{2 +} + {H}^{+}}$

You should be given $\textsf{p {K}_{a}}$ for this. My Cotton and Wilkinson gives $\textsf{p {K}_{a} = 4.95}$.

For a weak acid a quick way to get the pH is to use the expression:

sf(pH=1/2[pK_a-loga]

Where a is the initial concentration of the solution.

$\therefore$$\textsf{p H = \frac{1}{2} \left[4.95 - \left(- 0.3\right)\right]}$

$\textsf{p H = 2.62}$

Small, highly charged cations like this tend to be quite acidic.