Question

The chemical equation below shows the burning of magnesium (Mg) with oxygen (O2) to form magnesium oxide (MgO).

2Mg + O2 Right arrow. 2MgO

The molar mass of O2 is 32.0 g/mol. What mass, in grams, of O2 is required to react completely with 4.00 mol of Mg?
2.00 grams
64.0 grams
128 grams
256 grams

Answer 1

From the balanced equation 2Mg + O2 → 2MgO, 2 mol Mg react with 1 mol O2 (Mg:O2 = 2:1).
For 4.00 mol Mg, moles O2 = 4.00 × (1/2) = 2.00 mol.
Mass O2 = 2.00 mol × 32.0 g/mol = 64.0 g.

Answer: 64.0 grams.