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Question

A metal carbonate containing an unknown +2 metal ("MCO3") reacts with excess acid resulting in the loss of CO2. Analysis of the experimental data indicates that the mass % CO2 in the unknown metal carbonate is 30%. Which of the metal cations below is most likely to be the unknown metal in the original metal carbonate?

a.Pb2+

b.Sr2+

c.Ca2+

d.Cu2+

Answer 1

MCO₃ + 2H⁺ ===> M²⁺ + CO₂ + H₂O balanced equation

Since the molar mass of CO2 is 44 g/mole, find a metal carbonate where 44/molar mass = 0.30.

So, 44/x = 0.3 and x = 147 g/mol as a molar mass of the metal carbonate.

Since CO₃^2- has a molar mass of 60, the metal would have an atomic mass of 147 - 60 = 87.

This would be strontium (Sr²⁺) Choice (b)

To check this out, look at SrCO3

Sr = 87

C = 12

O = 16 x 3 = 48

Molar mass = 147 g/mole

If you had 100 g of SrCO3, you'd have 0.68 mole

1 mole SrCO₃ produces 1 mole CO₂ so there's be 0.68 mole CO₂

0.68 mol CO₂ x 44 g/mole = 29.9 g CO₂

29.9 g CO₂/100 g SrCO₃ = 29.9% CO₂ (close enough)