Question

If Ka for HNO2,=4.5x10-4, find the [H+] a 0.9 M HNO2 solution.

Answer 1

......HNO2 ==> H^+ + NO2^-
I.....0.9......0......0
C.....-x.......x......x
E....0.9-x.....x......x

Substitute the E line into Ka expression for HNO2 and solvle for H^+

Answer 2

You can also use for weak acids in pure water the 'Sqr Root' formula...
[H^+] = Sqr=Root(Ka·[weak acid])

[H^+] = Sqr=Root[(4.5x10^-4)(0.90)]M = 0.02M

for weak bases...
[OH^-] Sqr=Root(Ka·[weak base])