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Difficulty: EasyLimiting and Excess Reactants in Chemical Reactions
Hydrogen gas reacts with oxygen gas to form water according to the balanced chemical equation:
2H2(g)+O2(g)→2H2O(g)2\text{H}_{2(g)} + \text{O}_{2(g)} \rightarrow 2\text{H}_2\text{O}_{(g)}
If 4.0 moles4.0\text{ moles} of hydrogen gas (H2\text{H}_2) are mixed with 3.0 moles3.0\text{ moles} of oxygen gas (O2\text{O}_2) and allowed to react completely, which of the following correctly identifies the limiting reactant and the amount of excess reactant remaining?
  1. H2\text{H}_2 is the limiting reactant, and 1.0 mole1.0\text{ mole} of O2\text{O}_2 remains unreacted.Answer
  2. B
    O2\text{O}_2 is the limiting reactant, and 1.0 mole1.0\text{ mole} of H2\text{H}_2 remains unreacted.
  3. C
    H2\text{H}_2 is the limiting reactant, and 2.0 moles2.0\text{ moles} of O2\text{O}_2 remain unreacted.
  4. D
    O2\text{O}_2 is the limiting reactant, and 2.0 moles2.0\text{ moles} of H2\text{H}_2 remain unreacted.

Answer

H2\text{H}_2 is the limiting reactant, and 1.0 mole1.0\text{ mole} of O2\text{O}_2 remains unreacted.
The balanced chemical equation indicates a 2:12:1 mole ratio between H2\text{H}_2 and O2\text{O}_2. Reacting 4.0 moles4.0\text{ moles} of H2\text{H}_2 requires 2.0 moles2.0\text{ moles} of O2\text{O}_2. Since 3.0 moles3.0\text{ moles} of O2\text{O}_2 are present, H2\text{H}_2 is completely consumed first (limiting reactant), leaving 1.0 mole1.0\text{ mole} of O2\text{O}_2 unreacted as excess.

Step-by-Step Solution

1
Determine the mole ratio from the balanced equation
The mole ratio of H2\text{H}_2 to O2\text{O}_2 is 2:12 : 1.
The stoichiometric coefficients in 2H2+O2→2H2O2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} dictate that 2 moles2\text{ moles} of H2\text{H}_2 consume 1 mole1\text{ mole} of O2\text{O}_2.
2
Calculate the required moles of oxygen for the given hydrogen
Required moles of O2=4.0 moles H2×1 mole O22 moles H2=2.0 moles O2\text{Required moles of } \text{O}_2 = 4.0\text{ moles } \text{H}_2 \times \frac{1\text{ mole } \text{O}_2}{2\text{ moles } \text{H}_2} = 2.0\text{ moles } \text{O}_2.
This determines how much oxygen is actually needed to completely react with all 4.0 moles4.0\text{ moles} of hydrogen.
3
Identify the limiting reactant and excess amount
Available O2=3.0 moles\text{O}_2 = 3.0\text{ moles}, which is greater than the required 2.0 moles2.0\text{ moles}. Therefore, H2\text{H}_2 is the limiting reactant and excess O2=3.0−2.0=1.0 mole\text{O}_2 = 3.0 - 2.0 = 1.0\text{ mole}.
The reactant that runs out first (H2\text{H}_2) limits the reaction, leaving unreacted excess oxygen.

Key Concept

Limiting Reactant Determination
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Topics

Limiting and Excess Reactants in Chemical Reactions

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