Using only the periodic table, a calculator, and the information below, identify

Question

Using only the periodic table, a calculator, and the information below, identify the strong electrolyte whose general formula is M x (A) y • z H2O

Ignore the effect of interionic attractions in the solution

a. A n- is a common oxyanion. When 30.0 mg of the anhydrous sodium salt containing this oxyanion ( NanA , where n = 1, 2, or 3) is reduced in a reaction that involves the gain of 2 moles of electron per mole of oxyanion, 15. 26 mL of 0.02313 M reducing agent is required to react completely with the NanA present.

b. The cation is derived from a silvery white metal that is relatively expensive. The metal itself crystallizes in a body centered cubic unit cell and has an atomic radius of 198.4 pm. The solid pure metal has a density of 5.243 g/cm3 . The oxidation number of M in the strong electrolyte in question is +3.

c. When 33.4 mg of the compound is present (dissolved) in 10.0 mL of aqueous solution at 25C, the solution has an osmotic pressure of 558 torr.

Explanation / Answer

a. From this data

moles of reducing agent used = 0.02313 M x 15.26 ml = 0.353 mmol

molar mass of A = 30 x 2/0.363 = 165.29 g/mol

A could be BrO4- ion

b. From this data

atomic radius = 198.4 pm = 1.984 x 10^-8 cm

Edge length of unit cell = 16(1.984 x 10^-8)^2/9 = 2.64 x 10^-8 cm

Volume of unit cell = (2.64 x 10^-8)^3 = 1.84 x 10^-23 cm^3

Mass of 2 atoms in bcc unit cell = 1.84 x 10^-23 x 5.243 = 9.65 x 10^-23 g

mass of 1 atom of M = 9.65 x 10^-23 x 6.023 x 10^23/2 = 29.061 g/mol

M could be Phosphorous

c. molarity of compound = (558/760)/4 x 0.08205 x 298 = 0.0075 M

molar mass of compound = 0.0334/0.0075 x 0.01 = 444.92 g/mol

formula of the compound could be thus P(BrO4)3

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