What are the major species present in 0.250 M solutions of each of the following acids?

Question

Calculate the pH of the solution: NH4Cl

Answer 1

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The major species are NH4+ and Cl-.

Because NH4+ is a weak acid, there will be a much lower concentration of H+ in the solution as well.

NH4+ <--> H+ + NH3

Ka = [H+][NH3]/[NH4+] = 5.6X10^-10

Let [H+] = [NH3] = x, and [NH4+] = 0.250-x. Because Ka is very small, x will be insignificant compared to 0.250 and can be ignored. Then,

Ka = 5.6X10^-10 = x^2 / 0.250

x = 1.2X10^-5 M = [H+]

pH = -log [H+] = 4.93