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Difficulty: MediumChemical Reactions and Compounds

In residential heating, propane gas (C3H8\text{C}_3\text{H}_8) burns in the presence of oxygen (O2\text{O}_2) to form carbon dioxide (CO2\text{CO}_2) and water vapor (H2O\text{H}_2\text{O}) according to the unbalanced chemical equation:

C3H8+‾ O2→3CO2+4H2O\text{C}_3\text{H}_8 + \underline{\hspace{0.8cm}}\,\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}

What coefficient must be placed before O2\text{O}_2 to completely balance this equation using the lowest whole numbers?

  1. A
    10
  2. 5Answer
  3. C
    7
  4. D
    3

Answer

5
The coefficient 5 is correct because the product side contains a total of 10 oxygen atoms (6 from 3CO23\text{CO}_2 and 4 from 4H2O4\text{H}_2\text{O}). Since molecular oxygen is diatomic (O2\text{O}_2), 5 molecules of O2\text{O}_2 (5×2=105 \times 2 = 10 atoms) are required to balance the reaction.

Step-by-Step Solution

1
Count the total number of oxygen atoms on the product side of the equation.
The products contain (3×2)+(4×1)=6+4=10(3 \times 2) + (4 \times 1) = 6 + 4 = 10 oxygen atoms.
By the Law of Conservation of Mass, the number of oxygen atoms in the reactants must equal the number in the products.
2
Determine the coefficient needed for diatomic oxygen gas (O2\text{O}_2) on the reactant side.
10÷2=510 \div 2 = 5
Because each O2\text{O}_2 molecule contains 2 oxygen atoms, 5 molecules are required to provide the 10 needed atoms.

Key Concept

Balancing chemical equations by conserving atoms across reactants and products
Estimated Time:1m 0s
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Topics

Chemical Reactions and Compounds

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