Calculate the $pH$ of

a$.$ a $5\mathrm{.}23{10}^{-3}$ molar phenol $(H{C}_6{H}_{5}O)$ solution $)=(1\mathrm{.}6{10}^{-10}.$

b$.0\mathrm{.}625M$ ethyl amine $(C{H}_{3}N{H}_2)$ solution $)=(4\mathrm{.}2{10}^{-4}$