Question

Assertion (A): Following is a balanced chemical equation for the action of steam on iron: $3\mathrm{Fe}\left(\mathrm{s}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)\to {\mathrm{Fe}}_3{\mathrm{O}}_4\left(\mathrm{s}\right)+4{\mathrm{H}}_2\left(\mathrm{g}\right)$

Reason (R): The law of conservation of mass holds good for a chemical equation.

• A. Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
• B. Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
• C. (A) is true, but (R) is false.
• D. (A) is false, but (R) is true.

Answer 1

The correct option is A

Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).

Explanation for assertion:-

• When iron reacts with steam the products that are formed are Iron (II, III) oxide and hydrogen gas.
• The complete reaction is:

$\underset{\mathrm{Iron}}{3\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{Steam}}{4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)}\to \underset{\mathrm{Iron}(\mathrm{II},\mathrm{III})\mathrm{oxide}}{{\mathrm{Fe}}_3{\mathrm{O}}_4\left(\mathrm{s}\right)}+\underset{\mathrm{Hydrogen}\mathrm{gas}}{4{\mathrm{H}}_2\left(\mathrm{g}\right)}$

• Here the number of moles of atoms on both sides (reactant and product) is the same.
• Therefore, the chemical equation is balanced.
• Thus the assertion is true.

Explanation for reason:-

• Law of conservation of mass states that mass can neither be created nor be destroyed, it can only be transformed from one form to another in a chemical reaction.
• Thus for a chemical equation to obey the law of conservation of mass, it has to be balanced.
• Here in the reaction, the number of moles of atoms on both sides (reactant and product) is the same.

$\underset{\mathrm{Iron}}{3\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{Steam}}{4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)}\to \underset{\mathrm{Iron}(\mathrm{II},\mathrm{III})\mathrm{oxide}}{{\mathrm{Fe}}_3{\mathrm{O}}_4\left(\mathrm{s}\right)}+\underset{\mathrm{Hydrogen}\mathrm{gas}}{4{\mathrm{H}}_2\left(\mathrm{g}\right)}$

• Therefore, the mass of the reactants is the same as the mass of the products.
• Thus law of conservation of mass holds good for a chemical equation.
• Thus the reason is also true and is the correct explanation for the assertion.

Therefore, option (A) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A) is the correct answer.