Question

$1.0g$ of magnesium is burnt with $0.56g$ of $O_2$ in a closed vessel. Which reaction is left in excess and how much? (Atomic weight of $Mg=24;O=16$)

• A. $Mg,0.16g$
• B. $O_2,0.16g$
• C. $Mg,0.44g$
• D. $O_2,0.28g$

Answer 1

The correct option is A $Mg,0.16g$

The balanced chemical reaction is shown below:
$Mg+\frac{1}{2}{O}_2\to MgO$

Moles: $\frac{1.0}{24};\frac{0.56}{32}$
$\frac{0.5}{12};\frac{0.07}{4}$
$\frac{0.5}{12}-x;\frac{0.07}{4}-\frac{x}{2}$
Oxygen is limiting reagent so, $\frac{0.07}{4}-\frac{x}{2}=0$
$x=\frac{0.07}{2}$
Excess $Mg=\frac{0.5}{12}-\frac{0.07}{2}$ mol
Mass of $Mg$ is $=1-0.7\times 12=0.16$ g
Thus, when $1.0$ g of magnesium is burnt with $0.56$ g $O_2$ in a closed vessel, $0.16$ g magnesium is left in excess.