A 5g mass of magnesium metal reacts with a 4.0g mass of oxygen gas. Which reagent is in stoichiometric excess?

1 Answer
Feb 21, 2017

We follow the stoichiometric equation:

2Mg(s)+O2(g)2MgO(s)

Explanation:

Moles of metal=5.0g24.3gmol1=0.206mol

Moles of dioxygen=4.0g32.0gmol1=0.125mol

Dioxygen gas is in stoichiometric excess, and only 0.103 mol of dioxygen will react. And thus 0.022mol dioxygen will remain unreacted, i.e. approx. 0.7g.