Get real answers from millions of real people.?

Question

I am supposed to calculate the pH of a solution that is .49 M NH4ClO4. Can anybody help me?

Answer 1

NH4+ is a weak acid:

NH4+ <=> NH3 + H+

Ka = [H+][NH3]/[NH4+] (from definition) = 5.7 x 10^-10 (from tables)

Call [H+] y (just to give it a name)

Every time you make H+, you make NH3, so [NH3] = y

Every time you make H+, you lose NH4+.So in this case [NH4+] = 0.49 M - y

Substituting, 5.7 x 10^-10 = y^2/(0.49 - y)

EITHER expand and solve the quadratic (serous masochists only)

OR say y is very small COMPARED WITH 0.49. So, near enough,

0.49 x 5.7 x 10^-10 = y^2

Take square roots, find y (which is the same as [H+], remember?), and now you know [H+] you can easily find pH