CHM112 Home Page Reference Materials Homework Assignments Lectures Exams & Quizzes Grades Study Aids URI Home Page


J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 15.73

Calculate [NH4+] in a solution that is 0.15 M NH3 and 0.015 M KOH.






Answer

Write the chemical reactions (the solubility of KOH and the acid–base equilibrium for NH3), write the mass action expression for Kb (ammonia is a base), set up a table of concentrations, and solve.


KOH(aq)
K+(aq)
+
OH(aq)
NH3(aq)
+
H2O(l)
NH4+(aq)
+
OH(aq)
Initial
0.15
0
0.015
Change
– x
+ x
+ x
Equilibrium
0.15 – x
x
0.015 + x

Approximate? 0.15/1.8×10–5 = 8000 > 100, yes; 0.015/1.8×10–5 = 800 > 100, also yes.

x = [NH4+] = 1.8×10–4 M