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 reagent will be left.
A. ${O_2}$ is the limiting reagent and $Mg$ is in excess by $0.25g$.
B. $Mg$ is the limiting reagent and is in excess by $0.5g$.
C. ${O_2}$ is the limiting reagent and is in excess by $0.25g$.
D. ${O_2}$ is the limiting reagent and $Mg$ is in excess by $0.75g$.

Answer 1

Limiting reagent: The reagent which is completely consumed during the reaction, is known as limiting reagent.
Excess reagent: The reagent which is left after the reaction, is known as excess reagent.

Complete step by step answer:
Complete step by step solution:
First of all we will talk about reagents and products.
Reagents: those compounds which react with each other to form the other compounds, are known as reagents. They are present on the left side of the reaction.
Products: The compounds formed by the reaction of reagents, are known as products. They are present on the right side of the reaction.
Now we will talk about limiting reagents, excess reagent and balanced reaction.
Limiting reagent: The reagent which is completely consumed during the reaction, is known as limiting reagent.
Excess reagent: The reagent which is left after the reaction, is known as excess reagent.
Balanced reaction: The reaction in which the number of all atoms on both sides i.e. on the reactant side and on the product side are same, are known as balanced reactions.
In this question it is given that $1g$ of $Mg$ is burnt in a closed vessel containing $0.5g$ of ${O_2}$.
The balanced reaction of burning of magnesium is as: $2Mg + {O_2} \to 2MgO$
From the reaction, it is clear that two moles of magnesium reacts with one mole of oxygen to form two moles of magnesium oxide. In the question we are given that $1g$ of magnesium i.e. moles will be $\dfrac{{{\text{mass}}}}{{{\text{molar mass}}}}$i.e. $\dfrac{1}{{24}} = 0.0411moles$ reacts with $0.5g$ of ${O_2}$ i.e. moles will be $\dfrac{{0.5}}{{32}} = 0.0156$.
So $0.0156$ moles of oxygen will react with $0.0156 \times 2 = 0.0312$ moles of magnesium. And the remaining magnesium will be left $0.0411 - 0.0312 = 0.099$ moles. So the mass of magnesium left will be $0.099 \times 24 = 0.25grams$. And oxygen is limiting reagent

So, the correct answer is Option A .

Note:
Number of moles is defined as the ratio of mass of substance to the molar mass of the substance. In the balanced chemical reaction we can get the information about how much moles will react with how much moles of other reactants to form the product of given quantity.