Q.The concentration of hydroxide ion in a water sample is found to be $2.5 \times 10^{-6}$ M. Identify the nature of the solution.

Acid/base solutions typically span the concentration range $10^{-1}$ to $10^{-7}$ M, an inconveniently wide span. Sorensen's logarithmic pH scale expresses this compactly: $pH=-\log_{10}[H_3O^+]$, so $[H_3O^+]=10^{-pH}$; the analogous quantity for hydroxide is $pOH=-\log_{10}[OH^-]$.

pH + pOH = pKw. Adding the two definitions gives $pH+pOH=-\log_{10}([H_3O^+][OH^-])=-\log_{10}K_w=pK_w$. At $25^\circ C$, $K_w=1\times10^{-14}$, so $pK_w=14$ and $pH+pOH=14$. In a neutral solution at $25^\circ C$, both pH and pOH equal 7; acidic solutions have pH < 7, basic solutions have pH > 7. …