[tex]\text{ Cu\lparen NO}_3)_2\text{ }\rightarrow\text{ Cu}^{2+}\text{ + 2NO}_3^-[/tex]
The NO3- ion is the conjugate base of a strong acid, it will not affecy the pH, but Cu2+ ion is not conjugate acid of a strong base, it will affect the pH.
[tex]\text{ Cu}^{2+}\text{ + 2H}_2\text{O }\rightleftarrows\text{ Cu\lparen OH\rparen}_2\text{ + 2H}^{+\text{ }}\text{ Ka=3.00x10}^{-8}^{\frac{}{}}[/tex][tex]\text{ Ka= }\frac{\lbrack\text{ H}^+\rbrack^2}{\lbrack\text{ Cu}^{2+}\rbrack}[/tex]
we can use the approximation that [Cu2+] = initial concentration of Cu(NO3)2 because the is Ka is very small
Solve for [H+]:
[tex]\begin{gathered} \lbrack\text{ H}^+\rbrack\text{ = }\sqrt{\text{ Ka }\times\text{ }\lbrack\text{ Cu}^{2+}\rbrack} \\ \lbrack\text{ H}^+\rbrack\text{ = }\sqrt{3.00\times10^{-8}\text{ }\times\text{ 1.30}}=\text{ 1.97}\times10^{-4} \end{gathered}[/tex][tex]\begin{gathered} \text{ pH = }-\text{ log \lparen}\lbrack\text{H}^+\rbrack) \\ \text{ pH = - log \lparen1.97}\times10^{-4})\text{ = 3.71 } \end{gathered}[/tex]