SOLUTION:
Step 1: Write the balanced chemical equation.
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Step 2: Use dimensional analysis to calculate the mass of ethanol combusted at STP.
Notes:
• Based on the balanced chemical equation, 1 mole of C₂H₅OH is stoichiometrically equivalent to 2 moles of CO₂.
• At STP, the volume of 1 mole of an ideal gas is 22.414 L.
• The molar mass of ethanol is 46.07 g/mol.
[tex]\begin{aligned} \text{mass of} \: \text{C}_2\text{H}_5\text{OH} & = 20.4 \: \cancel{\text{L}} \times \frac{1 \: \cancel{\text{mol} \: \text{CO}_2}}{22.414 \: \cancel{\text{L}}} \times \frac{1 \: \cancel{\text{mol} \: \text{C}_2\text{H}_5\text{OH}}}{2 \: \cancel{\text{mol} \: \text{CO}_2}} \times \frac{\text{46.07 g} \: \text{C}_2\text{H}_5\text{OH}}{1 \: \cancel{\text{mol} \: \text{C}_2\text{H}_5\text{OH}}} \\ & = \boxed{\text{21.0 g}} \end{aligned}[/tex]
Hence, the mass of ethanol combusted is 21.0 g.
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