Question

A) Determine the pH of a 0.98 x 10^-2 mol L solution of hydrocyanic acid (HCN) Ka = 4.0 x 10^-10

B) What is the pH of a 0.243mol L solution of methylamine? (pKb for CH3NH2 = 3.30)

C) determine the pH of a buffer solution of Na2CO3 (pkb = 3.68, 0.125 M) and NaHco3 (0.35M)

Answer 1

A.
...........HCN ==> H^+ + CN^-
I.......0.0098.....0......0
C.........-x........x.....x
E......0.0098-x.....x.....x

Substitute the E line into the Ka expression and solve for x = (H^+) then convert to pH.

B.
.......CH3NH2 + HOH ==> CH3NH3^+ + OH^-
Set up an ICE chart as in A above, substitute into Kb and solve for x = OH^-, then convert to pH.

C.
Use the Henderson-Hasselbalch equation.
pH = pKa + log (base)/(acid)
You know pKb, convert to pKa.
You know M base (Na2CO3) and M acid(NaHCO3)
Post your work if you get stuck on any of these.