Discussion
Calculate the pH of an aqueous ammonia solution that has \(\mathrm{OH^{-}}\) concentration of \(1.9 \times 10^{-3}\) M.
Answer: A
The pH is 11.28.
First find pOH from the hydroxide concentration, then use pH + pOH = 14 at 25 °C.
\[ \begin{aligned} \mathrm{pOH} &= -\log(1.9 \times 10^{-3}) \\ &= 3 - 0.28 = 2.72 \\ \mathrm{pH} &= 14 - 2.72 = 11.28 \end{aligned} \]
An ammonia solution is basic, so its pH must exceed 7; 5.29 would be acidic. 9.28 and 7.28 do not follow from this concentration: they would need an hydroxide concentration a hundred or ten thousand times smaller.
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