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Mass-Volume Stoichiometry

Determining the amount of gas or the material needed to react with a gas in a reaction

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Mass-Volume and Volume-Mass Stoichiometry

Credit: Image copyright Sony Ho, 2014
Source: http://www.shutterstock.com/

How much azide is needed to fill an air bag?

Cars and many other vehicles have air bags in them. In case of a collision, a reaction is triggered so that the rapid decomposition of sodium azide produces nitrogen gas, filling the air bag. If too little sodium azide is used, the air bag will not fill completely and will not protect the person in the vehicle. Too much sodium azide could cause the formation of more gas that the bag can safely handle. If the bag breaks from the excess gas pressure, all protection is lost.

Mass to Volume and Volume to Mass Problems

Chemical reactions frequently involve both solid substances whose mass can be measured as well as gases for which measuring the volume is more appropriate. Stoichiometry problems of this type are called either mass-volume or volume-mass problems.

Because both types of problems involve a conversion from either moles of gas to volume or vice-versa, we can use the molar volume of 22.4 L/mol provided that the conditions for the reaction are STP.

Sample Problem: Mass-Volume Stoichiometry

Aluminum metal reacts rapidly with aqueous sulfuric acid to produce aqueous aluminum sulfate and hydrogen gas.

\begin{align*}2 \text{Al}(s)+3\text{H}_2\text{SO}_4 (aq) \rightarrow \text{Al}_2 (\text{SO}_4)_3 (aq)+3\text{H}_2 (g)\end{align*}

Determine the volume of hydrogen gas produced at STP when a 2.00 g piece of aluminum completely reacts.

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

Known

• given: 2.00 g Al
• molar mass Al = 26.98 g/mol
• 2 mol Al = 3 mol H 2

Unknown

• volume H 2 = ?

The grams of aluminum will first be converted to moles. Then the mole ratio will be applied to convert to moles of hydrogen gas. Finally, the molar volume of a gas will be used to convert to liters of hydrogen.

\begin{align*}\text{g Al} \rightarrow \text{mol Al} \rightarrow \text{mol H}_2 \rightarrow \text{L H}_2\end{align*}

Step 2: Solve.

\begin{align*}2.00 \text{ g Al} \times \frac{1 \text{ mol Al}}{26.98 \text{ g Al}} \times \frac{3 \text{ mol H}_2}{2 \text{ mol Al}} \times \frac{22.4 \text{ L H}_2}{1 \text{ mol H}_2} = 2.49 \text{ L H}_2\end{align*}

The volume result is in liters. For much smaller amounts, it may be convenient to convert to milliliters. The answer here has three significant figures. Because the molar volume is a measured quantity of 22.4 L/mol, three is the maximum number of significant figures for this type of problem.

Sample Problem: Volume-Mass Stoichiometry

Calcium oxide is used to remove sulfur dioxide generated in coal-burning power plants according to the following reaction.

\begin{align*}2\text{CaO}(s) + 2\text{SO}_2 (g)+\text{O}_2 (g) \rightarrow 2\text{CaSO}_4 (s)\end{align*}

What mass of calcium oxide is required to react completely with 1.4 × 10 3 L of sulfur dioxide?

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

Known

• given: 1.4 × 10 3  L = SO 2
• 2 mol SO 2 = 2 mol CaO
• molar mass CaO = 56.08 g/mol

Unknown

• mass CaO = ? g

The volume of SO 2 will be converted to moles, followed by the mole ratio, and finally a conversion of moles of CaO to grams.

\begin{align*}\text{L SO}_2 \rightarrow \text{mol SO}_2 \rightarrow \text{mol CaO} \rightarrow \text{g CaO}\end{align*}

Step 2: Solve.

\begin{align*}1.4 \times 10^3 \text{ L SO}_2 \times \frac{1 \text{ mol SO}_2}{22.4 \text{ L SO}_2} \times \frac{2 \text{ mol CaO}}{2 \text{ mol SO}_2} \times \frac{56.08 \text{ g CaO}}{1 \text{ mol CaO}}=3.5 \times 10^3 \text{ g CaO}\end{align*}

The resultant mass could be reported as 3.5 kg, with two significant figures. Even though the 2:2 mole ratio does not mathematically affect the problem, it is still necessary for unit conversion.

Summary

• Calculations are described for determining the amount of gas formed in a reaction.
• Calculations are described for determining amounts of a material needed to react with a gas.

Review

1. What are the conditions for all gases in these calculations?
2. How can you tell if all the ratios were set up correctly?
3. Why was 2 mol CaO/2mol SO 2 included in the second example if it did not affect the final number?