Upon completion of this topic, learners will be able to:

• apply the concept of the gas laws to gas phase reactions and perform stoichiometric calculations.

In this section, we use the ideal-gas equation first to define the relationship between the density of a gas and its molar mass, and then to calculate the volumes of gases formed or consumed in chemical reactions.

Molar mass of a gas

Recall that density of a gas was measured to determine the molar mass of the gas molecules

Stoichiometric problems involving gas volumes

Consider the following reaction, which is often used to generate small quantities of oxygen gas:

Suppose you heat 0.0100 mol of potassium chlorate, KClO₃, in a test tube. How many liters of oxygen can you produce at 298 K and 1.02 atm?

You solve such a problem by breaking it into two problems, one involving stoichiometry and the other involving the ideal gas law. Note that 2 mol KClO₃ yields 3 mol O₂. Therefore,

Now that you have the moles of oxygen produced, you can use the ideal gas law to calculate the volume of oxygen under the conditions given. Rearrange the ideal gas law, PV = nRT, and solve for the volume:

Substitute for n, T, and P. Because the pressure is given in units of atmospheres, you choose the value of R in units of L·atm/(K·mol). The answer comes out in liters.