Question

A buffer solution contains 0.263 M CH3NH3Cl and 0.339 M CH3NH2 (methylamine). Determine the pH change when 0.076 mol HNO3 is added to 1.00 L of the buffer. pH after addition − pH before addition = pH change =

A buffer solution contains 0.333 M NaH₂PO₄ and 0.221 M Na₂HPO₄. Determine the pH change when 0.046 mol HClO₄ is added to 1.00 L of the buffer.

pH change =

Answer 1

Q1.

this is a buffer so

pOH = pKb + log(CH3NH3Cl /CH3NH2 )

pKb = 3.38 for methyl amine, CH3NH2

initially

mol of base = MV = 0.339*1 = 0.339

mol of conjugate = MV = 0.263*1 = 0.263

after additino of HNO3:

mol of base = MV = 0.339 - 0.076 = 0.263

mol of conjugate = MV = 0.263+0.076 =0.339

so

pOH = pKb + log(CH3NH3Cl /CH3NH2 )

pOH = 3.38 + log(0.339/0.263)

pOH = 3.490

pH = 14-3.490 = 10.51

change from original:

pH original = 14 - (3.38 + log(0.263/0.339) = 10.73

dpH = 10.73-10.51

decrease in 0.22 pH units

Q2.

similarly:

pH = pKa2 + log(HPO4-2 /H2PO-)

pH = 7.21 + log((0.221-0.046)/(0.333+0.046))

pH = 6.8743

change in pH:

pH initial = 7.21 + log(0.221/0.333) = 7.0319

dpH = 7.0319-6.8743 = 0.1576 drops