Question

$1g$ of $Mg$ is burnt in a closed vessel containing $0.5g$ of $O_2$. Which reactant is limiting how much of the reagent and excess reactant will be left?

• A. $O_2$ is a limiting reagent and $Mg$ is in excess by $0.25g$
• B. $Mg$ is a limiting reagent and is in excess by $0.5g$
• C. $O_2$ is a limiting reagent and is in excess by $0.25g$
• D. $O_2$ is a limiting reagent and $Mg$ is in excess by $0.75g$

Answer 1

Answer: (A)

$O_2$ is a limiting reagent and $Mg$ is in excess by $0.25g$

Balanced reaction is:

$2Mg+O_2\to 2MgO$

According to the reaction 2 mol (or $2\times 24.3=48.6$g) Mg reacts with 32 g oxygen. 1 g of Mg requires $=\frac{32}{48.6}=0.66$g oxygen but given oxygen is 0.5 g $\therefore$ oxygen is the limiting reagent

0.5 g oxygen can burn =$=\frac{48.6}{32}\times 0.5=0.75$g Mg

Excess of Mg left =$1-0.75=0.25g$