Stoichiometry equations

Question

2 Mg + O₂ → 2 MgO

Using the above balanced chemical equation, calculate how many moles of O₂ are needed to produce 3.49 moles of MgO.

Answer 1

2Mg + O2 → 2MgO

? mol O2 = 3.49 mol MgO this is what the problem is asking

? mol O2 = 3.49 mol MgO × (1 mol O2)/(2 mol MgO) = 1.75 mol O2

1. multiply the given moles of MgO by the ratio of O2 :MgO from the balanced equation
2. mol MgO cancels out to leave mol O2