College Preparatory Program • Saudi Aramco
Acid-Base Titration Problem
Acid-Base Titration Problem
CH3NH2(aq) + H2O(l)  CH3NH3+(aq) + OH-(aq) Kb = 4.38 x 10-4
In aqueous solution of methylamine at 25C, the hydroxide ion concentration is 1.50 x 10-3
M. In answering the following, assume that temperature is constant and that volumes are
additive.
(a) Write the equilibrium constant expression for the reaction above.
(b) Determine the initial concentration of methylamine.
(c) Determine the percent ionization of methylamine in the above solution.
(d) What is the number of moles of HCl that should be added to 100. mL of 0.100 M CH3NH2
to produce a solution buffered at pH = 10.125.
(e) 125 .0 ml of 0.200 M CH3NH2HCl is titrated with 0.100 M NaOH to the equivalence point
and beyond the equivalence point.
Find the pH of a 0.200 M CH3NH2HCl solution.
Find the volume of NaOH needed to reach the equivalence point.
Find the pH at the equivalence point.
Sketch a rough curve for the above titration on the graph below and indicate the
initial pH, the pH at half-way to the equivalence point and the pH at the equivalence
point.
14
12
10
pH
(i)
(ii)
(iii)
(iv)
8
6
4
2
0
0
50
100
150
200
250 300
350
volume of NaOH ( mL)
400
450
500