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Answer 1 · 2017-01-17T13:56:52

You will require a sample of impure $CaC_2$ that is 10.02 g in mass.

The equation for acetylene production is

$CaC_2 + 2H_2O rarr C_2H_2 + Ca(OH)_2$

At STP, 3.36 L of any gas represents $3.36-:22.4 = 0.15 mol$

Since the ratio between $CaC_2$ and $C_2H_2$ is 1 to 1 in the equation, we require 0.15 mol of calcium carbide to do the job.

The molar mass of $CaC_2$ is 64.1 g, so 0.15 mol would have a mass of 9.615 g.

Since the sample is 4% impure, the 9.615 g must represent 96% of the mass of the impure sample, so the sample must be

$9.615 -: 0.96 = 10.02 g$ in size.