Question detail
In the reaction 2H2 + O2 → 2H2O, if 4 g of hydrogen reacts completely, calculate the mass of water produced. (Relative atomic mass: H = 1, 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
Amount of substance
Question
In the reaction 2H2 + O2 → 2H2O, if 4 g of hydrogen reacts completely, calculate the mass of water produced. (Relative atomic mass: H = 1, O = 16)
Answer
36 g. This answer is anchored to Balanced equations and reacting quantities.
Explanation
Use Moles From Mass: moles = mass / Mr. The worked result is 36 g. 36 g is the best answer because it directly supports the AQA A-Level Chemistry objective to use balanced equations to calculate reacting masses. This reasoning is anchored to Balanced equations and reacting quantities in Amount of substance, and it separates balanced equation from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.
Common mistake
Common Mistake in Reacting Mass Calculations
Students often forget to convert units when using balanced equations to calculate reacting masses, leading to incorrect results.
Always ensure that all quantities are in the correct units before performing calculations. For example, if using grams, make sure to convert to moles using the molar mass if necessary. Use the formula: mass = moles × molar mass. Substitute the values and calculate to find the correct mass in grams.
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