[10%] An unknown salt MX2 is a group 2 metal halide. — Solutions and Colligative Properties Chemistry Question
Problem Context
[10%] An unknown salt MX2 is a group 2 metal halide.
10.00 g MX2 dissolves in 50.0 g water to give a homogeneous solution. The freezing point of this solution is –4.50 °C. What is the molar mass of MX2? For water, Kf = 1.86 °C/m.
Model Answer
(–4.50 °C)/(–1.86 °C/m) = 2.42 m
Since MX2 gives 3 moles of ions per mole of compound, the solution is (2.42 m/3) = 0.807 m.
(0.807 mol MX2/kg water) × (0.0500 kg water) = 0.0404 mol MX2
10.00 g MX2/0.0404 mol = 248 g mol-1
10.00 g Na2CO3 and 10.00 g MX2 are mixed in 200.0 mL of water. A precipitate of MCO3 forms. What is the pH of the supernatant? The Ka of H2CO3 is 4.3 × 10-7 and the Ka of HCO3– is 4.7 × 10-11
Model Answer
10.00 g Na2CO3/(105.99 g mol-1) = 0.09435 mol Na2CO3.
After reaction with 0.0404 mol MX2, approximately 0.0404 mol MCO3 will precipitate, leaving behind 0.0540 mol CO32- in solution. In 200 mL, this gives a 0.270 M solution of CO32-. The carbonate ion reacts with water as follows:
CO32-(aq) + H2O(l) ⇌ HCO3–(aq) + OH–(aq) Keq = Kw/(Ka of HCO3–)
[HCO3-][OH-]/[CO32-] = 2.1 × 10-4
[OH-]2/[0.270] = 2.1 × 10-4
[OH–] = 7.6 × 10-3 M
pH = 14 + log10[OH–] = 11.88
A solution of 10.00 g MX2 in water is treated with excess silver nitrate. The precipitate is dried; the mass of the dried compound is 15.2 g. What is the identity of MX2?
Model Answer
15.2 g AgX/(0.0404 • 2 mol X) = 188 g mol-1 of AgX
Since the atomic mass of Ag is 107.9, the molar mass of X is 80 ⇒ X is Br.
Subtracting the mass of 2 Br from 248 g mol-1 molar mass of MX2 gives an atomic mass of M = 88. Thus M = Sr.
A sample of 10.00 g MX2 dissolved in 50 mL water is treated with increasing amounts of Na2SO4 up to 10 g in total. How will the mass of precipitate formed vary with the mass of added Na2SO4? Graph your answer on the grid provided.
Model Answer
0.0404 mol SrSO4 × 183.69 g mol-1 = 7.42 g SrSO4 is the maximum amount of precipitate that can form. The amount actually formed will increase linearly until 0.0404 mol Na2SO4 (= 5.74 g) are added, then will not increase further as Sr2+ becomes the limiting reagent.
What color flame test does MX2 give?
Model Answer
Sr gives a red flame test.