Question

Buffer pH by Design

Given that Kb = 1.8×10-5 for ammonia: To prepare exactly 650 mL of a buffer solution which has a pH of 9.47 and a final ammonium chloride concentration of 0.140 M, one would need to use what (mass) of ammonium chloride, and what (volume) of 2.50 M sodium hydroxide solution, plus enough water to make up the total volume.

Answer 1

moles of NH4Cl = molarity x volume

                         = 0.140 x 0 .650

                        = 0.091

molar mass of NH4Cl = 53.5 g / mol

mass of NH4Cl = moles x molar mass

                         = 0.091 x 53.5

                       = 4.87 g

mass of ammonium chloride = 4.87 g

pOH = 14- 9.47

pOH = 4.53

pKb = -log (1.8 x 10^-5)

pKb = 4.74

NH4Cl   + NaOH ------------------> NH4OH + NaCl

pOH = pKb + log [NH4Cl / NH4OH]

4.53 = 4.74 + log (0.091 / x)

x = 0.148

moles of NH4OH = moles of NaOH = 0.148

molarity = moles / volume

2.50 = 0.148 / volume

volume = 0.0590 L

volume = 59.0 mL

volume of NaOH = 59.0 mL

volume of water = 650 - 59

                             = 591 mL