Calculate the mass of calcium oxide that was formed when firing 100 g of limestone (caco3) containing 25% impurities.

Given:

m (CaCO3) = 100 g

w% (impurities) = 25%

To find:

m (CaO) -?

Decision:

CaCO3 = CaO + CO2, – we solve the problem, relying on the composed reaction equation:

1) Find the mass of pure limestone that has reacted:

100% – the entire mass of the mixture,

25% – impurities,

then the proportion of pure limestone is 100% – 25% = 75%.

m (CaCO3) = 100 g * 0.75 = 75 g

2) Find the amount of limestone:

n (CaCO3) = m: M = 75 g: 100 g / mol = 0.75 mol

3) We compose a logical expression:

If 1 mol of CaCO3 gives 1 mol of CaO,

then 0.75 mol CaCO3 gives x mol CaO,

then x = 0.75 mol.

4) Find the mass of calcium oxide:

m (CaO) = n * M = 0.75 mol * 56 g / mol = 42 g.

Answer: m (CaO) = 42 g.