Question

Calculate the pH of the solution, when 50ml of 2.0HNO2(ka=4×10^-4) titrated with 50ml of 1.0MKOH

Answer 1

....................HNO2 + KOH ==> KNO2 + H2O

initial...........100 mmols....50........0............0
change........-50...............-50......+50..........50
equilibrium....50..................0........50..........50
What you have after the reaction is a buffer; i.e., you have a weak acid (HNO2) and its salt (KNO2) so use the Henderson-Hasselbalch equation.
pH = pKa + log [(KNO2)/(HNO2)]
Plug in the numbers and solve for pH. Post your work if you get stuck.