In the reaction 2H2 + O2 gives 2H2O, 4 moles of hydrogen are mixed with 3 moles of oxygen. The amount of the excess reactant left over is
Correct answer: D. 1 mole of oxygen
- A. 2 moles of hydrogen
- B. 3 moles of oxygen
- C. nothing, both are exactly consumed
- D. 1 mole of oxygen
Explanation
Four moles of hydrogen require only two moles of oxygen, so hydrogen is limiting and 3 minus 2, that is 1 mole of oxygen, remains unreacted. Four moles of water are produced. The point of the calculation is that the excess is found by subtracting what was actually used from what was supplied, not by comparing the starting amounts directly.
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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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