Discussion
The pH of a solution is 4 then it's OH- ion concentration (in mol dm-3) is
Answer: B
The hydroxide ion concentration is \(10^{-10}\) mol dm-3.
At 25 °C, pH + pOH = 14 (from \(K_{w} = [\mathrm{H^{+}}][\mathrm{OH^{-}}] = 10^{-14}\)):
\[ \begin{aligned} \mathrm{pOH} &= 14 - 4 = 10 \\ [\mathrm{OH^{-}}] &= 10^{-10}\ \text{mol dm}^{-3} \end{aligned} \]
\(10^{-4}\) is the hydrogen ion concentration, not the hydroxide. The solution is acidic, so \([\mathrm{OH^{-}}]\) must be much smaller than \(10^{-7}\), which also rules out \(10^{-2}\) and \(10^{-6}\).
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