Question

The amount of calcium in a 15.0-g sample was determined by converting the calcium to calcium oxalate, CaC2O4. The CaC2O4 weighed 11.0 g. What is the percent of calcium in the original sample?

a.

9.4 %

b.

22.9 %

c.

12.9 %

d.

29.4 %

e.

73.3 %

Answer 1

Molar mass of CaC2O4,
MM = 1*MM(Ca) + 2*MM(C) + 4*MM(O)
= 1*40.08 + 2*12.01 + 4*16.0
= 128.1 g/mol


mass(CaC2O4)= 11.0 g

use:
number of mol of CaC2O4,
n = mass of CaC2O4/molar mass of CaC2O4
=(11.0 g)/(128.1 g/mol)
= 8.587*10^-2 mol
This is number of moles of CaC2O4

one mole of CaC2O4 contains 1 moles of Ca
use:
number of moles of Ca = 1 * number of moles of CaC2O4
= 1 * 8.587*10^-2
= 8.587*10^-2

Molar mass of Ca = 40.08 g/mol

use:
mass of Ca,
m = number of mol * molar mass
= 8.587*10^-2 mol * 40.08 g/mol
= 3.442 g

mass % of Ca = mass of Ca * 100 / mass of sample
= 3.442 * 100 / 15.0
= 22.9 %

Answer: 22.9 %