What volume of gaseous reactant produces a given amount of product?

What steps do I need to take to solve this problem?

The brominated alkane ethylene dibromide, or 1,2-dibromoethane is a common but controversial fumigant and is also an anti-knock addditive in gasolines. It is prepared by the action of bromine on ethene gas. How many liters of ethane at 35°C and 3.00 atmospheres are required to prepare 500.0 kg of ethylene dibromide, assuming the reaction goes to completion?
-Ryan Cramer

This is a reaction stoichiometry problem, since you're trying to relate an amount of one substance to an amount of another through a chemical reaction. Here is a general strategy for solving such problems:

1. Pick out the target. You want liters of ethane.
2. List the given information. You have 500.0 kg of ethylene dibromide. The ethane used to make it is at 35°C and 3.00 atm. Your strategy is to convert kg of ethylene dibromide into L of ethane gas:
   500.0 kg C₂H₄Br₂ L C₂H₆(g)

3. Search for relationships that connect the given information with the target. This is usually the most difficult step. It helps to work backwards from the target towards the given information. Also, whenever an amount of one substance is being used to obtain an amount of another, you need a mole-to-mole ratio that relates the two substances. Knowing that lets you break the problem into three easier pieces:
   500.0 kg C₂H₄Br₂ mol C₂H₄Br₂ mol C₂H₆ L C₂H₆(g)
   The kg C₂H₄Br₂ to mol C₂H₄Br₂ conversion will require the molar mass of C₂H₄Br₂. The mol C₂H₄Br₂ to mol C₂H₆ conversion requires a balanced chemical equation for the reaction (although, if you look at the formulas, you'll realize that every mole of C₂H₄Br₂ formed requires one mole of C₂H₆). Finally, to compute L C₂H₆(g) from moles of C₂H₆, you'll need to think about how to compute the volume of a gas given the moles, pressure, and temperature of the gas...
4. Check your answer. Before you pick up your calculator, decide roughly how many of liters of ethane you expect to get. Whenever you can, you should use a different method to check your calculation.

Author: Fred Senese senese@antoine.frostburg.edu

Copyright © 1997-2010 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/gases/faq/gas-stoichiometry-ethane-ethylene-bromide.shtml