Stoichiometry help

Question

Ethanol, C₂H₅OH, is considered clean fuel because it burns in oxygen to produce carbon dioxide and water with few trace pollutants. If 150.0 g of H₂O are produced during the combustion of ethanol, how many grams of ethanol were present at the beginning of the reaction? 

Answer 1

First you must balance your chemical reaction:

1C₂H₅OH + 6 O₂ --> 2 CO₂ + 3 H₂O

Then you must calculate the molar mass of water (H₂O) and ethanol(C₂H₅OH) using your periodic table.

H₂O = 18.02g/mol

C₂H₅OH = 47.07g/mol

With this information you can start doing stoichiometry to cancel out your units.

150g of H₂O divided by 18.01g/moles = 8.33 moles of H₂O

8.33moles of H₂O multiplied by 1mole of C₂H₅OH/ 3 moles of H₂O = 2.78 moles of C₂H₅OH

2.78 moles of C₂H₅OH multiplied by 46.07g/mol of C₂H₅OH = 128.07 grams of C₂H₅OH