Chemistry Question

Question

Ethanol, C₂H₅OH, is considered clean fuel because it burns in oxygen to produce carbon dioxide and water with few trace pollutants. If 500.0 g of H₂O are produced during the combustion of ethanol, how many grams of ethanol were present at the beginning of the reaction? 

1. Have both the unbalanced and balanced chemical equations.
2. Explain how to find the molar mass of the compounds.
3. Explain how the balanced chemical equation is used to find the ratio of moles
4. Explain how many significant figures your answer needs to have.
5. The numerical answer 

Answer 1

1. C₂H₅OH + O₂ ==> CO₂ + H₂O ... unbalanced equation
2. C₂H₅OH + 3O₂ ==> 2CO₂ + 3H₂O ... balanced equation
3. The balanced equation tells us that the mole ratio of H₂O produced to C₂H₅OH used is 3 : 1
4. The answer should have 4 significant figures because the data given (500.0 g H2O) has 4 sig.figs.
5. To find the numerical answer we will convert the 500.0 g of H₂O to moles of H₂O, and then use the 3:1 mole ratio of the balanced equation to find the moles of C₂H₅OH used. Finally, we'll convert mols C₂H₅OH to grams by using the molar mass of 46.07 g / mol.

500.0 g H₂O x 1 mol H₂O / 18.02 g = 27.747 mols H₂O

27.747 mols H₂O x 1 mol C₂H₅OH / 3 mols H₂O = 9.2490 mols C₂H₅OH

9.2490 mols C₂H₅OH x 46.07 g / mol = 426.1 g C₂H₅OH (4 sig.figs.)