Question

$30\, g$ of magnesium and $30 \,g$ of oxygen are reacted, then the residual mixture contains

• A. $60 \,g$ of magnesium oxide only
• B. $40 \,g$ of magnesium oxide and $20 \,g$ of oxygen
• C. $45 \,g$ of magnesium oxide and $15\, g$ of oxygen
• D. $50 \,g$ of magnesium oxide and $10 \,g$ of oxygen

Answer 1

Answer: (D)

$\underset{2\,mol}{2\,Mg}+\underset{1\,mol}{O_{2}} \to \underset{2\,mol}{2MgO}$

$30 \,g$ of $Mg =\frac{30}{24}=1.25 \,mol$ and $30 \,g$ of $O _{2}=\frac{30}{32}$

$=0.9375\, mol$

$1.25 \,mol$ of $Mg$ will react with $\frac{1.25}{2}=0.625$ mole of oxygen

Thus magnesium is the limiting reagent and will be consumed completely.

Remaining oxygen $ =0.9375-0.625=0.3125 \,mol$

$ =0.3125 \times 32=10\, g$

$MgO$ formed $=1.25\, mol \equiv 1.25 \times 40=50 \,g$