Question

calculate the pH of a solution containing .25M nitrous acid (Ka=4.5x10^-40
calculate the pH of a solution containing .25M sodium nitrite
calculate the pH of a solution containing .25M nitrous acid and .75M sodium nitrite

Answer 1

Ka=4.5x10^-4 ie pKa = 3.35

Answer 2

The first is a standard weak-acid equilibrium problem. Get out your ICE table.

In the second case, you have the conjugate base of the weak acid in the first problem, so you have a standard weak base equilibrium problem. Same approach.

In the third case, you have a buffer, so you can take short circuit and use the Henderson-Hassellbach equation. By the way, your Ka is wrong, looks like you typed a 0 instead of a ).