Question

In: Chemistry

Equal volumes of 0.14 mol L–1 NH3 (Kb = 1.8 x 10–5) and 0.14 mol L–1...

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

Solutions

Expert Solution

                   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


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