Chemistry - Ionic Equilibrium Question with Solution | TestHub

Question Explanation:

Calculate the pH of a weak base solution using its dissociation constant $\left({\mathrm{K}}_{\mathrm{b}}\right)$.

Concept:

ionic Equilibrium ( pH of weak base)

Solution:

${\mathrm{CH}}_3{\mathrm{NH}}_2$ is a weak base. We first calculate the concentration of hydroxide ions $\left[{\mathrm{OH}}^{-}\right]$find the pOH and then determine the pH using the given ion product of water $\left({\mathrm{pK}}_{\omega }=13\right)$.

Calculation:

$\left[{\mathrm{OH}}^{-}\right]=\sqrt{{\mathrm{K}}_{\mathrm{b}}\cdot \mathrm{C}}=\sqrt{2\times 10^{-6}\times 2}=\sqrt{4\times 10^{-6}}=2\times 10^{-3}\mathrm{M}$ $\mathrm{pOH}=-\log \left[{\mathrm{OH}}^{-}\right]=-\log \left(2\times 10^{-3}\right)=3-\log 2=3-0.3=2.7$ $\mathrm{pH}={\mathrm{pK}}_{\omega }-\mathrm{pOH}=13-2.7=10.3$

Final Answer: 10.3