Question

2. You are trying to make 1.0 liter a pH 11.5 solution of Ammonia NH3 (Kb= 1.8 *10 -5), how many grams do you need to measure out? (hint, first find the concentration of the solution and then figure out grams.)

Answer 1

equilibrium reaction involved is;

NH₃ + H₂O <--->NH₄⁺ + OH^-

so at equilibrium we will be having [NH₄⁺]=[OH^-]

expressions for K_b = [NH₄⁺][OH^-]/[NH₃]

so [NH₃]=[NH₄⁺][OH^-]/K_b

given pH=11.5

so pOH=14-pH= 2.5

[OH^-]=10^-2.5=[NH₄⁺]

on solving we will get [NH₃]=0.55M

so no. of moles of ammonia in 1L= 0.55 moles

mass of ammonia = 0.55*17 = 9.44g