more stoichiometry

Question

If 200.8 g of carbon dioxide gas is made from the complete combustion of isopropanol (C3H7OH), how many moles of oxygen are used up?

Answer 1

First, you must write a correctly balanced equation:

2C₃H₇OH + 9O₂ ==> 6CO₂ + 8H₂O ... balanced equation

The stoichiometry tells us that 9 moles of O₂ will be used up for every 6 moles of CO₂ formed.

We are given the grams of CO₂ formed, so we MUST convert to moles of CO₂

moles CO₂ = 200.8 g CO₂ x 1 mol CO₂ / 44 g = 4.564 moles CO₂

Find moles of O₂:

4.564 moles CO₂ x 9 moles O₂ / 6 mol CO₂ = 6.845 moles O₂ used up