Question

1.0 g of $Mg$ is burnt with 0.28 g of $O_2$ in a closed vessel. Which reactant is left in excess and by how much?

• A. $Mg$, 5.8 g
• B. $Mg$, 0.58 g
• C. $O_2$, 0.24 g
• D. $O_2$, 2.4 g
  

Answer 1

The correct option is B $Mg$, 0.58 g

The burning of Mg occurs as follows:
$2Mg+O_2⟶2MgO$
Moles 2 1 2
Molar mass 48 32 -
32 g of oxygen, needs 48 g of Mg
0.28 g of oxygen, needs =$\frac{48\times 0.28}{32}$= 0.42 g of Mg
Thus, Mg left (in excess) $=1-0.42=0.58g$