Question

What stoichiometric concentration of the indicated substance is required to obtain an aqueous solution with the pH value shown below?
a)aniline, C6H5NH2, for pH= 9.05
b)NH4Cl for pH= 5.22

Answer 1

a) Aniline is a base.
pH = 9.05; pOH = 4.95; OH^- = 1.12E-5M
......C6H5NH2 + HOH ==> C6H5NH3^+ + OH^-
initial..x................0..........0
change..-1.12E-5......1.12E-5..1.12E-5
equil..x-1.12E-5....1.12E-5...1.12E-5

Kb = (C6H5NH3^+)(OH^-)/(C6H5NH2)
Substitute into the Kb expression, look up Kb for aniline, solve for x = M aniline for pH of 9.05

b) The pH is determined by the hydrolysis of the salt. pH = 5.22; (H^+) = 6.03E-6
........NH4^+ + H2O ==> H3O^+ + NH3
initial...x...............0.......0
change..-6.03E-6.....6.03E-6..6.03E-6
equil..x-6.03E-6......6.03E-6..6.03E-6

Ka for NH4^+ = (Kw/Kb for NH3) = (NH3)(H3O^+)/(NH4^+)

Substitute into the Ka expression from above and solve for x = (NH4+) = (NH4Cl)
initial.

Answer 2

When I do a) I get an answer of 0.298M, however, it says I'm wrong. b) worked perfectly

Answer 3

I obtained 0.299 but you have used 4.2E-10 for Kb which is just two significant figures. I would round the 0.298 you obtained to 0.30 and submit that. I would appreciate letting me know if this takes care of the problem. This post is getting buried so just make a new question to DrBob222 and let me know. Thanks.