Problem $5-16($modified$),$ Davis and Cornwell, Introduction to Environmental Engineering, $5$th Ed$.$:

Determine the concentration $(mo\frac{l}{L})$ of hydroxide that must be added to an aqueous solution to produce a concentration of $0\mathrm{.}50m\frac{g}{L}$ copper if the starting concentration is $3\mathrm{.}5m\frac{g}{L}$ copper. The solubility product for copper hydroxide $(Cu(OH{)}_2)$ is $2\mathrm{.}2{10}^{-20}mo\frac{l}{L}.$ What is the resulting solution $pH?$

Problem $3\mathrm{.}15($modified$),$ Mihelcic and Zimmerman, Environmental Engineering: Fundamentals, Sustainability, Design, $2$nd Ed$.$:

When $C{l}_2$ gas is added to water during the disinfection of drinking water, it hydrolyzes with the water to form HOCl. The disinfection power of the acid HOCl is much better than its conjugate base $OC{l}^{-}.$ The $p{K}_a$ for HOCl is $7\mathrm{.}5.$ What percentage of the total disinfection power $($:$OC{l}^{-}\}$exists in the acid form $(HOCl)$ at $pH=6\mathrm{.}75$ and at $pH=7\mathrm{.}25?$