Law Of Conservation Of Mass

Question

When 10.5 g of iron filings are burned completely in excess oxygen, 15.0 g of iron oxide ($\text{Fe}_2\text{O}_3$) is formed. What mass of oxygen was consumed in the reaction?

RankGuru · Accepted Answer

The Law of Conservation of Mass, established by Lavoisier, states that the total mass of reactants equals the total mass of products in a chemical reaction — matter is neither created nor destroyed. Applying this law: mass of iron $+$ mass of oxygen $=$ mass of iron oxide. Therefore, mass of oxygen $=$ mass of iron oxide $-$ mass of iron $= 15.0 - 10.5 = 4.5$ g. Option 3.0 g is incorrect; it is the difference between 15.0 and 12.0, which does not correspond to any quantity given. Option 5.5 g would imply the product mass is 16.0 g, not 15.0 g. Option 6.0 g would imply $\text{Fe} + \text{O}_2$ masses of $10.5 + 6.0 = 16.5$ g, contradicting the given product mass. This question tests the NCERT fundamental law of conservation of mass, which underpins all stoichiometric calculations. Plausibility check: the amount of oxygen (4.5 g) is a reasonable fraction of the iron mass (10.5 g), consistent with the formula $\text{Fe}_2\text{O}_3$ where oxygen accounts for $48/160 = 30\%$ of the mass, and $4.5/15.0 = 30\%$.

Law Of Conservation Of Mass

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