In: Chemistry
Equal volumes of 0.14 mol L–1 NH3 (Kb = 1.8 x 10–5) and 0.14 mol L–1 HCl are mixed. Calculate the pH of the resulting solution.
(A) 2.9
(B) 5.0
(C) 5.2
(D) 6.2
(E) 7.8
NH3 + HCl ------------> NH4Cl
no of moles of NH3 = 0.14moles
no of moles of Hcl = 0.14 moles
equal volume of NH3 and HCl
total volume = V+ V = 2v = 2
molairy of NH4^+ = 0.14/2 = 0.07 M
NH4^+ + H2O -------------. NH3 + H3O^+
I 0.07 0 0
C -x +x +x
E 0.07-x +x +x
Ka = Kw/Kb
= 1*10^-14/1.8*10^-5 = 5.6*10^-10
Ka = [NH3][H3O^+]/[NH4^+]
5.6*10^-10 = x*x/(0.07-x)
5.6*10^-10*(0.07-x) = x^2
x = 6.26*10^-6
[H3O^+] = x = 6.26*10^-6M
PH = -log[H3O^+]
= -log6.26*10^-6
= 5.2034 >>>>answer