How to prepare soln. and buffer soln.

Question

1. 25.0 mL 18 M NaOH solution with 5 g KI and 0.15 g NaN₃
2. 250.0 mL 1.0 M NH₃-NH₄⁺ pH 10 buffer solution from NH₄Cl and NH₃
3. 100.0 mL 0.0500 M stock Ca²⁺ solution

Answer 1

1).

molar mass NaOH = 40 g / mol

18 M NaOH = 18 mols NaOH / L x 0.025 L x 40 g / mol = 18 g NaOH needed

Weigh out 18 g NaOH. Weigh out 5 g KI. Weigh out 0.15 g NaN₃. Dissolve all in a total volume of 25.0 mls

2). NH₃ + H2O ==> NH4⁺ + OH-

Kb = [NH₄⁺][OH-] / [NH₃] = 2x10^-5 (looked up this value of Kb)

pH = 10, so pOH = 4, so [OH-] = 1x10^-4

[OH-] = Kb x [NH₃]/[NH₄⁺]

1x10^-4 = 2x10^-5 [NH₃]/[NH₄⁺]

[NH₃]/[NH₄⁺] = 5

Mix 208.5 mls of 1.0 M NH₃(aq) and 41.5 mls of 1.0 M NH₄Cl to get 250 mls of buffer with the appropriate ratio of NH₃ : NH₄⁺ and a pH of 10.

3). We need to have an anion associated with the Ca²⁺. A common salt would be CaCl₂, so I will use that.

molar mass CaCl₂ = 111 g / mol

100.0 mls x 1 L / 1000 mls = 0.1000 L

0.0500 mols / L x 0.1000 L x 111 g / mol = 0.555 g CaCl₂ needed to obtain 0.0500 M Ca²⁺