Question

PART A

How many grams of solid sodium cyanide should be added to 0.500 L of a 0.135 M hydrocyanic acid solution to prepare a buffer with a pH of 8.597 ?

grams sodium cyanide = _________ g.

PART B

How many grams of solid potassium fluoride should be added to 0.500 L of a 8.48×10^-2M hydrofluoric acid solution to prepare a buffer with a pH of3.807 ?

grams potassium fluoride = ________ g.

PART C

How many grams of solid ammonium bromide should be added to 0.500 L of a 8.61×10^-2 M ammonia solution to prepare a buffer with a pH of 8.270?

grams ammonium bromide = _________ g.

Answer 1

PART A

pH = 8.597

pKa = 9.21

moles of HCN = 0.5 x 0.135 = 0.0675

pH = pKa + log [salt / acid]

8.597 = 9.208 + log [NaCN / 0.0675]

moles of NaCN = 0.0165

mass of NaCN = 0.810 g

grams sodium cyanide = 0.810 g

part B)

pH = 3.807

pKa = 3.20

moles of HF = 0.5 x 8.48 x 10^-2 = 0.0424

pH = pKa + log [KF / HF]

3.807 = 3.20 + log [KF / 0.0424]

moles of KF = 0.172 mol

mass of potassium fluoride = 9.99 g

part C)

pH = 8.270

pOH = 5.73

pKb = 4.74

moles of NH3 = 0.5 x 0.0861 = 0.04305

pOH = pKb + log [NH4Br / NH3]

5.73 = 4.74 + log [NH4Br / 0.04305]

moles of NH4Br = 0.4207

mass of NH4Br = 41.2 g