Calculate the mass of magnesium if 256 g of oxygen has reacted.

Let’s find the amount of substance O2:

M (O2) = 32 g / mol.

n = 256 g: 32 g / mol = 8 mol.

Let’s find the quantitative ratios of substances.

2Mg + O2 = 2MgO.

For 1 mole of O2, there are 2 moles of Mg.

Substances are in quantitative ratios 1: 2.

The amount of Mg substance is 2 times greater than the amount of O2 substance.

n (Mg) = 2n (O2) = 8 × 2 = 16 mol.

Let’s find the mass Mg.

M (Mg) = 24 g / mol.

m = n × M.

m = 24 g / mol × 16 mol = 384 g.

Answer: 384