a,
$CH_3COOH\rightleftharpoons CH_3COO^-+H^+$
$[H^+]=x$
$\Rightarrow [CH_3COO^-]=x; [CH_3COOH]=0,1-x (M)$
$K_a=1,75.10^{-5}$
$\Rightarrow \dfrac{x^2}{0,1-x}=1,75.10^{-5}$
$\Leftrightarrow x=1,31.10^{-3}$
$\to pH=-\log_{10}x=2,88$
b,
$K_b=\dfrac{10^{-14}}{1,8.10^{-5}}=5,55.10^{-10}$
$NH_3+H_2O\rightleftharpoons NH_4^++OH^-$
$[OH^-]=x\Rightarrow [NH_4^+]=x; [NH_3]=0,1-x (M)$
$\Rightarrow \dfrac{x^2}{0,1-x}=5,55.10^{-10}$
$\Leftrightarrow x=7,45.10^{-6}$
$\to pH=14-pOH=14+\log_{10}x=8,87$
