Question

How many moles of oxygen are required to completely combust 1 mole of propane (C\(_3\)H\(_8\))?

• A. 4 moles
• B. 5 moles
• C. 6 moles
• D. 3 moles

Hint:

Use the shortcut \(x + y/4\) to quickly find the moles of \(O_2\) for any hydrocarbon \(C_xH_y\). For propane: \(3 + 8/4 = 3 + 2 = 5\).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Combustion of a hydrocarbon involves its reaction with oxygen gas ($\text{O}_2$) to produce carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$). To find the required moles of oxygen, we need to write and balance the chemical equation for the combustion.
Step 2: Key Formula or Approach:
The general balanced equation for the complete combustion of an alkane ($\text{C}_n\text{H}_{2n+2}$) is: \[ \text{C}_n\text{H}_{2n+2} + \left( \frac{3n + 1}{2} \right) \text{O}_2 \rightarrow n \text{CO}_2 + (n+1) \text{H}_2\text{O} \] Step 3: Detailed Explanation:
For propane ($\text{C}_3\text{H}_8$), $n = 3$.
Let's balance the equation step by step without the formula to be sure: \[ \text{C}_3\text{H}_8 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} \] 1. Balance Carbon: There are 3 C atoms on the left, so we need 3 $\text{CO}_2$ on the right. \[ \text{C}_3\text{H}_8 + \text{O}_2 \rightarrow 3\text{CO}_2 + \text{H}_2\text{O} \] 2. Balance Hydrogen: There are 8 H atoms on the left, so we need 4 $\text{H}_2\text{O}$ on the right. \[ \text{C}_3\text{H}_8 + \text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O} \] 3. Balance Oxygen: There are $(3 \times 2) + (4 \times 1) = 6 + 4 = 10$ oxygen atoms on the right side. Therefore, we need 5 $\text{O}_2$ molecules on the left side. \[ \text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O} \] From the balanced equation, 1 mole of propane reacts with 5 moles of oxygen.
Step 4: Final Answer:
5 moles of oxygen are required. The correct option is (B).