What mass of methanol is combusted in a reaction that produces 112 L of Co2 at STP?
A. 112g
B. 81.6g
C. 64.1g
D. 160g

Answer

D 160g

Explanation:

Write the equation:

Combustion reactions use oxygen and release water and heat, so

  CH₃OH(g) + O₂(g) → CO₂(g) + H₂O(g)

Balance that:

  2CH₃OH(g) + 3O₂(g) → 2CO₂(g) + 4H₂O(g)

Find moles of carbon dioxide:

We need to know the number of moles of CO₂. This rxn is at STP, so at STP one mole of gas = 22.4 liters.

  112 L * 1 mol/22.4 L = 5 mol CO₂

Find moles of methanol:

Based on the chemical equation, for every 2 mol methanol, there are 2 mol carbon dioxide. So for every 5 mol carbon dioxide, there are 5 mol methanol!

  5 mol CO₂ = 5 mol CH₃OH

Molar mass of methanol: 12.01 + 3*1.008 + 16.00 + 1.008 = 32.04 g/mol

Moles of methanol: 5 mol * 32.04 g/mol = 160.2 g methanol

≈ 160 mol methanol

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