Question

30 g Mg and 30 g ${{\text{O}}_{\text{2}}}$ are reacted and the residual mixture contains:
A. 60 g of MgO only
B. 40 g of MgO and 20 g of ${{\text{O}}_{\text{2}}}$
C. 45 g of MgO and 15 g of ${{\text{O}}_{\text{2}}}$
D. 50 g of MgO and 10 g of ${{\text{O}}_{\text{2}}}$

Answer 1

Hint: To answer this question, we should first find the limiting reagent. We should start the question with first writing the reaction and then going further.

Step by step answer:
So, let us now start the answer of this question with the reaction of magnesium and oxygen.
Reaction between magnesium and oxygen:
$\text{Mg}+{{\text{O}}_{\text{2}}}\to \text{MgO}$
We will first start with balancing the equation.
$\text{2Mg}+{{\text{O}}_{\text{2}}}\to \text{2MgO}$
Magnesium molecular mass= 24$\times$2=48 gram
Oxygen molecular mass= 16$\times$2=32 gram
Magnesium oxide molecular mass (2MgO) = 2$\times$40 gram=80 gram
Now, we can say that 48g of Mg combined with 32 g of oxygen give 80g of MgO.
In this reaction, oxygen is the limiting reagent. Let us discuss limiting reagent. We should know that limiting reagent in a chemical reaction is the substance that is totally consumed when the chemical reaction is completed. The amount of product formed is limited by this reagent, since the reaction cannot continue without it. We should know that the reactant that produces a lesser amount of product is the limiting reagent. The reactant that produces a larger amount of product is the excess reagent.
As we know that oxygen is a limiting reagent.
30 gram of ${{\text{O}}_{\text{2}}}$will give $\to \left( \dfrac{80}{48}\times 30 \right)MgO$
So, now we can say that 30 gram $O_2$ will form 75 gram MgO.
So, now we can say that residual oxygen or unreacted oxygen is= 30-20=10gram
Now, we can say that MgO formed is 50 gram.
So, we can say that option D is the correct answer.

Note: We should note that stoichiometry is the quantitative relation between the number of moles (and therefore mass) of various products and reactants in a chemical reaction. Chemical reactions must be balanced, or in other words, must have the same number of various atoms in the products as in the reactants.