$1)$$NH_4OH+HCl\rightarrow NH_4Cl+H_2O$
$n_{HCl}>n_{NH_4OH}\Rightarrow n_{H^+}\approx n_{HCl}-n_{NH_4OH}=...=2.10^{-3} mol$
$\Rightarrow [H^+]=\frac{2.10^{-3}.1000}{25+35}=\frac{1}{30}\Rightarrow pH=-lg(\frac{1}{30})=...$
$2)$
$\begin{cases}pH_1=3 \\ pH_2=4 \end{cases}\Rightarrow \begin{cases}[H^+]_1=10^{-3} \\ [H^+]_2=10^{-4} \end{cases}\Rightarrow V_1.10^{-3}=V_2.10^{-4}\Rightarrow \frac{V_2}{V_1}=10$
Suy ra pha loãng $10$ lần