We know that $\mathrm{p}K_{\mathrm{a}} + \mathrm{p}K_{\mathrm{b}} = 14$. Given that $\mathrm{p}K_{\mathrm{b}}$ of $\mathrm{NH}_{3}$ is 4.75, we can find $\mathrm{p}K_{\mathrm{a}}$ by subtracting $\mathrm{p}K_{\mathrm{b}}$ from 14.
\[\mathrm{p}K_{\mathrm{a}} = 14 -
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