Question

​50 ml of 0.2 m ammonia solution is treated with 25 ml of 0.2 m hcl. If pkb of ammonia solution is 4.75, the ph of the mixture will be : (1) 3.75 (2) 4.75 (3) 8.25 (4) 9.25 50 ml of 0.2 m ammonia solution is treated with 25 ml of 0.2 m hcl. If pk b of ammonia solution is 4.75, the ph of the mixture will be

Answer 1

pH will be 9.25.

You need to consider the following calculations in details

the equation for the reaction is HCl (aq]  +  NH ₃(aq]  →  NH ₄⁺(aq]  +  Cl ⁻ (aq]

moles of NH3 aq is 50 x 0.2/1000 =0.01 moles

moles of HCl is 25 x 0.2/1000 = 0.005 moles

so limiting reactant is HCl

then moles of NH3 reacting completely is 0.01- 0.005 =0.005 moles

so the total volume will be 50 + 25 = 75ml

concentration = 0.005/0.075 = 0.07 M

Kb = 1.8 x 10^-5

pOH =pkb + log( [conjugate acid]/[weak base])

but we have equal concentrations of conjugate acid and weak base and thus its calculation amounts to 0

pOH= -log .8 x 10^-5 =4.74

Then pH= 14 - 4.74

         pH= 9.25