1.0 g of $\mathrm{Mg}$ is burnt with 0.56 g ${\mathrm{O}}_2$  in a closed vessel.  Which reactant is left in excess and how much?

24 g Magnesium requires 16 g of oxygen 

 0.56 g oxygen requires 0.84 g magnesium

 As 1g of Mg is taken, 

weight of magnesium left unreacted = 1 – 0.84 = 0.16g