Consider the reaction below, if 5 moles each of hydrogen and oxygen are reacted to form water, the reaction reveals:2H2 + O2 --> 2H2O
Correct answer: C. H2 is the limiting reagent
- A. H2 is the excess agent
- B. O2 is the limiting agent
- C. H2 is the limiting reagent
- D. The reaction has no limiting reagent
Explanation
Option C is the correct answer. In the reaction 2H2 + O2 --> 2H2O, we start with 5 moles of both hydrogen and oxygen. To find the limiting reagent, we need to calculate how many moles of water can be produced from each reactant:Moles of water from hydrogen: 5 moles H2 * (2 moles H2O / 2 moles H2) = 5 moles H2OMoles of water from oxygen: 5 moles O2 * (2 moles H2O / 1 mole O2) = 10 moles H2OSince hydrogen can produce only 5 moles of water while oxygen could produce 10 moles, hydrogen is the limiting reagent. This means that all of the hydrogen will be consumed in the reaction before all the oxygen is used up, leaving some oxygen unreacted. Thus, the maximum amount of product, which is water, that can be formed is 5 moles.The other options are incorrect because:Option A: Hydrogen is not the excess agent; it is the limiting reagent.Option B: Oxygen is not the limiting agent; it is present in excess.Option D: There is indeed a limiting reagent in this reaction.
This question appeared on the KMU MDCAT 2023 paper, which you can sit online with every answer explained.
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About Limiting and Excess Reactants
Limiting and excess reactants are identified from the balanced chemical equation and the available amounts of each substance. The limiting reactant determines the maximum product formed, while the excess reactant remains after completion, so questions require mole ratios, theoretical yield and sometimes percentage yield calculations.
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