How many kilograms of 80% phosphoric acid can be obtained from 31 kg of phosphorus containing 5% impurities?

Phosphoric acid has the formula H3PO4. One molecule contains one phosphorus atom. Therefore, the molar amount of phosphorus used and the resulting phosphoric acid will be equal.

Phosphorus contains 5% impurities. Therefore, the mass of pure phosphorus will be less and will be:

31 x (1 – 0.05) = 29.45 kg.

The molar amount of this mass of phosphorus is 29.45 x 31 = 912.95 mol (where 31 is the molar mass of phosphorus).

The same molar amount of phosphoric acid will be obtained.

Its molar mass is 1 x 3 + 31 + 16 x 4 = 98 grams / mol.

The mass of this acid will be equal to 98 x 912.95 = 89.4691 kilograms.

If this acid is diluted with water to 80%, then its mass will be 89.4691 / 0.8 = 111.836 kilograms.