What mass of ammonium chloride should be added to 2.65 L of a 0.165 M NH3 in order to obtain a buffer with a pH of 9.65?

Answer 1

For most buffer problems, you can use the Henderson Hasselbalch (HH) equation. There are two useful forms, one for acid buffers, and one for basic buffers.

acid buffer: pH = pKa + log [salt] / [acid]

base buffer: pOH = pKb + log [salt] / [base]

Since the current problem deals with a base buffer (NH3), we'll use that equation

pOH = pKb + log [NH₄⁺] / [NH₃]

Looking up the pKb for NH₃ we find a value of 4.75

Converting pH to pOH we have 14.00 - 9.65 = 4.35

pOH = pKb + log [NH₄⁺] / [NH₃]

4.35 = 4.75 + log [NH₄⁺] / [NH₃]

log [NH₄⁺] / [NH₃] = -0.40

[NH₄⁺] / [NH₃] = 0.398

[NH₄⁺] = 0.398 x 0.165

[NH₄⁺] = 0.0657 M

moles NH₄Cl needed: 2.65 L x 0.0657 mol / L = 0.174 moles NH₄Cl needed

mass NH₄Cl = 0.174 mols x 53.5 g / mol = 9.31 g NH₄Cl