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Gas Stoichiometry

The ideal gas law is used to balance equations involving gases

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Practice Gas Stoichiometry
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Gas Stoichiometry

How is fertilizer produced?

The Haber cycle reaction of gaseous nitrogen and hydrogen to form ammonia is a critical step in the production of fertilizer from ammonia. It is important to have an excess of the starting materials so that a maximum yield of ammonia can be achieved. By knowing how much ammonia is needed for manufacture of a batch of fertilizer, the proper amounts of nitrogen and hydrogen gases can be incorporated into the process.

Gas Stoichiometry

You have learned how to use molar volume to solve stoichiometry problems for chemical reactions involving one or more gases at STP. Now, we can use the ideal gas law to expand our treatment of chemical reactions to solve stoichiometry problems for reactions that occur at any temperature and pressure.

Sample Problem: Gas Stoichiometry and the Ideal Gas Law

What volume of carbon dioxide is produced by the combustion of 25.21 g of ethanol (C 2 H 5 OH) at 54°C and 728 mmHg? Assume the gas is ideal.

Before using the ideal gas law, it is necessary to write and balance the chemical equation. Recall that most combustion reactions, the given substance reacts with O 2 to form CO 2 and H 2 O. Here is the balanced equation for the combustion of ethanol.

$\text{C}_2\text{H}_5\text{OH}(l)+3\text{O}_2(g) \rightarrow 2\text{CO}_2(g)+3\text{H}_2\text{O}(l)$

Step 1: List the known quantities and solve the problem.

Known

• $\text{mass} \ \text{C}_2\text{H}_5\text{OH}=25.21 \text{ g}$
• $\text{molar mass} \ \text{C}_2\text{H}_5\text{OH}=46.08 \text{ g/mol}$
• $P=728 \text{ mmHg}$
• $T=54^\circ \text{C}=327 \text{ K}$

Unknown

• $\text{Volume} \ \text{CO}_2=? \text{ L}$

The number of moles of carbon dioxide gas is first calculated by stoichiometry. Then the ideal gas law is used to calculate the volume of CO 2 produced.

Step 2: Solve.

$25.21 \text{ g } \text{C}_2\text{H}_5\text{OH} \times \frac{1 \text{ mol } \text{C}_2\text{H}_5\text{OH}}{46.08 \text{ g } \text{C}_2\text{H}_5\text{OH}} \times \frac{2 \text{ mol } \text{CO}_2}{1 \text{ mol } \text{C}_2\text{H}_5\text{OH}}=1.094 \text{ mol } \text{C}_2\text{H}_5\text{OH}$

The moles of ethanol  $(n)$ is now substituted into  $PV=nRT$ to solve for the volume.

$V=\frac{nRT}{P}=\frac{1.094 \text{ mol} \times 62.36 \text{ L} \cdot \text{mmHg/K} \cdot \text{mol} \times 327 \text{ K}}{728 \text{ mmHg}}=30.6 \text{ L}$

The mass of ethanol is slightly more than one half mole, meaning that the mole ratio results in slightly more than one mole of carbon dioxide being produced. Because of the elevated temperature and reduced pressure compared to STP, the resulting volume is larger than 22.4 L.

Summary

• The ideal gas law is used to calculate stoichiometry problems for gases.

Practice

Solve the problems on the worksheet at this site:

Review

1. Do we need gas conditions to be at STP to calculate stoichiometry problems?
2. Why do we want to determine the stoichiometry of these reactions?
3. What assumption are we making about the gases involved?