Hello!
First, we need to determine the pKa of the base. It can be found applying the following equation:
[tex]pKa=14-pKb=14-3,35=10,65[/tex]
Now, we can apply the Henderson-Hasselbach's equation in the following way:
[tex]pH=pKa+log( \frac{[CH_3NH_2]}{[CH_3NH_3Cl]} )=10,65+log( \frac{0,18M}{0,73M} )=10,04[/tex]
So, the pH of this buffer solution is 10,04
Have a nice day!
