We’re being asked **to calculate the pH** of a **0.16 M CH _{3}NH_{3}Cl** using an ICE chart.

CH_{3}NH_{3}^{+} is a conjugate acid of the base CH_{3}NH_{2}. We need to use the K_{b} for CH_{3}NH_{2} to find the K_{a} of its conjugate acid, CH_{3}NH_{3}^{+}. Let us just omit Cl^{- }since it is neutral.

We can find on books and the internet that the K_{b} of CH_{3}NH_{2} is 4.4x10^{-4}. Therefore, the Ka of CH_{3}NH_{3}^{+} is:

$\overline{){{\mathbf{K}}}_{{\mathbf{w}}}{\mathbf{}}{\mathbf{=}}{\mathbf{}}{{\mathbf{K}}}_{{\mathbf{a}}}{{\mathbf{K}}}_{{\mathbf{b}}}}\phantom{\rule{0ex}{0ex}}{\mathbf{K}}_{{\mathbf{a}}}\mathbf{}\mathbf{=}\frac{\mathbf{}{\mathbf{K}}_{\mathbf{w}}}{{\mathbf{K}}_{\mathbf{b}}}\phantom{\rule{0ex}{0ex}}{\mathbf{K}}_{\mathbf{a}}\mathbf{}\mathbf{=}\frac{\mathbf{}\mathbf{1}\mathbf{.}\mathbf{0}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{14}}}{\mathbf{4}\mathbf{.}\mathbf{4}\mathbf{\times}{\mathbf{10}}^{\mathbf{-}\mathbf{4}}}$

**K _{a} = 2.27x10**

Since CH_{3}NH_{3}^{+} has a very low K_{a} value, it’s a weak acid. Remember that ** weak acids** partially dissociate in water and that

CH_{3}NH_{3}^{+}(aq) + H_{2}O(l) ⇌ H_{3}O^{+}(aq) + CH_{3}NH_{2}(aq); **K _{a} = 2.27×10^{–11}**

From this, we can construct an ICE table. Remember that liquids are ignored in the ICE table.

The ** K_{a} expression** for CH

$\overline{){{\mathbf{K}}}_{{\mathbf{a}}}{\mathbf{=}}\frac{\left[{H}_{3}{O}^{+}\right]\left[{\mathrm{CH}}_{3}{\mathrm{NH}}_{2}\right]}{\left[{\mathrm{CH}}_{3}{{\mathrm{NH}}_{3}}^{+}\right]}}$

*Liquids are ignored in the K _{a} expression.*

Note that each concentration is raised by the stoichiometric coefficient: [CH_{3}NH_{3}^{+}], [H_{3}O^{+}] and [CH_{3}NH_{2}] are raised to 1.

Solve an equilibrium problem (using an ICE table) to calculate the pH of each of the following solutions.

0.16 M CH_{3} NH_{3} Cl

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Weak Acids concept. If you need more Weak Acids practice, you can also practice Weak Acids practice problems.