Gas Stoichiometry

Balanced chemical equations can be used to relate moles or grams of reactant(s) to products. When gases are involved, these relations can be applied to include volume.

Avogadro’s Law

• “Equal volumes of all gases, at the same temperature and pressure, have the same number of molecules.”

Example of a Gas Stoichiometry Problem

Airbags in automobiles contain sodium azide (NaN3), potassium nitrate, and silicon dioxide. (All are solids.)

1. Upon impact, the bag is inflated by the thermal

decomposition of sodium azide (NaN3) to sodium

metal and nitrogen gas.

2. Because sodium is toxic and very reactive, it reacts with the potassium nitrate to produce potassium oxide and sodium oxide, and (additional) nitrogen gas.

3. The metal oxides are removed by reacting with the silicon dioxide to produce alkaline silicate (glass).

Question.

The driver’s airbag fills to 50-60 liters. Assuming the pressure inside the airbag is 1 atm, calculate the number of grams of each solid substance needed for a 50-L airbag.

When acid is added to sodium bicarbonate (sodium

hydrogen carbonate), NaHCO3, the following reaction

occurs:

NaHCO3(s) + H+(aq) Na+(aq) + H2CO3(aq)

but H2CO3(aq) quickly decomposes to CO2 + H2O,

so theactual reaction is:

NaHCO3(s) + H+(aq) Na+(aq) + CO2(g) + H2O(l)

All of the following experiments are performed with

2.45 M HCl and 12.75 g of NaHCO3 at 732 mm Hg and

38oC.

Example 5.6

2.45 M HCl and 12.75 g of NaHCO3 at 732 mm Hg and

38oC.

(a) If an excess of HCl is used, what volume of CO2 is

produced?NOTE:

The first solution that follows is ‘the long way around.’ After that, the solution is a shorter way that you should be able to do. The strategies are the same.

Strategy

Balanced chemical equation (1 NaHCO3 = 1 CO2)

(in pb.)

(i) Find amount of CO2 produced using stoichiometry

(ii) Static conditions so use PV = nRT

(note: you can’t use 22.4 L = 1 mol b/c it’s not at STP)

2.45 M HCl and 12.75 g of NaHCO3 at 732 mm Hg and 38oC.

(a) If an excess of HCl is used, what volume of CO2 is produced?NaHCO3(s) + H+(aq) Na+(aq) + CO2(g) + H2O(l)

2.45 M HCl and 12.75 g of NaHCO3 at 732 mm Hg and 38oC.

(a) If an excess of HCl is used, what volume of CO2 is produced?NaHCO3(s) + H+(aq) Na+(aq) + CO2(g) + H2O(l)

2.45 M HCl and 12.75 g of NaHCO3 at 732 mm Hg and

38oC.

(b) If NaHCO3 is in excess, what volume of HCl is

required to produce 2.65 L of CO2?

2.45 M HCl and 12.75 g of NaHCO3 at 732 mm Hg and

38oC.

(c) What volume of CO2 is produced when all of the

NaHCO3 is made to react with 50.0 mL HCl?Strategy

(i) This involves determining the limiting reagent (reactant).

(ii) Convert nCO2

NaHCO3(s) + H+(aq) Na+(aq) + CO2(g) + H2O(l)

2.45 M HCl and 12.75 g of NaHCO3 at 732 mm Hg and 38oC.

(c) What volume of CO2 is produced when all of the NaHCO3 is made to

react with 50.0 mL HCl?

N.B.

Solving the previous problems, I repeated many steps that you wouldn’t when solving the problem. For example, the number of moles was calculated in (a) and used again in (c). On the AP exam, you (and they) wouldn’t want you to repeat calculations.

Work smarter, not harder.