Question detail
In the reaction 2H2 + O2 → 2H2O, if you start with 4 grams of H2, calculate the mass of O2 needed to completely react with it. (Ar of H = 1, Ar of O = 16)
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
Question
Type
exam_style
Style
Topic
Use of amount of substance in relation to masses of pure substances
Question
In the reaction 2H2 + O2 → 2H2O, if you start with 4 grams of H2, calculate the mass of O2 needed to completely react with it. (Ar of H = 1, Ar of O = 16)
Answer
The answer is 8 g.
Explanation
This uses Moles From Mass because the objective is about (HT only) Calculate masses of reactants from a balanced symbol equation and a given mass of another substance. The reasoning belongs to Amounts of substances in equations (HT only) within Use of amount of substance in relation to masses of pure substances, so it should not be confused with nearby quantitative ideas such as mass, moles, concentration, yield, atom economy, or gas volume unless those are named in the objective. Use the focus term HT only to keep the answer aligned with AQA GCSE Chemistry 8462 Unit 4.3.
Common mistake
Confusing Mass and Moles
Students often confuse the mass of a substance with the number of moles when calculating the masses of reactants from a balanced symbol equation.
To fix this, remember to use the formula that relates mass, moles, and relative formula mass: mass = moles x Mr. Ensure you calculate the number of moles first before converting to mass. Keep the correction anchored to Amounts of substances in equations (HT only); check formula, substitution, calculation, final answer, and unit where relevant.
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