Chemical Stoichiometry and Gases: Using Ideal Gas Law to Determine Moles

Recall that for a balanced chemical reaction, calculations involving the mass of the reactants and products and the number of moles are performed by following a general conceptual plan.

Here, the stoichiometric coefficients are used as conversion factors between the moles of the reactant and the moles of the product.

In chemical reactions involving gaseous substances, the amount of gas is typically specified in terms of its volume at a given temperature and pressure. This is because gases are fluid and expand to fill any available volume.

In gaseous reactions, the mole amount and volume of a gas are interrelated through the ideal gas law. In this way, the number of moles can be determined from the volume of the gas and vice versa.

Combining concepts of the ideal gas law, molar mass, and stoichiometry allows for calculations related to the volume, the number of moles, and the mass of gaseous reactants and products.

As an example, consider the reaction between lithium and water to produce hydrogen gas. Assuming that the reaction occurs at 291 K and 0.977 atm, what amount of lithium will produce 35.25 liters of hydrogen?

First, by applying the ideal gas law and substituting the given values of pressure, volume, temperature, and the ideal gas constant, the moles of hydrogen gas are calculated.

Then, using the stoichiometric ratio, the number of moles of hydrogen gas is converted to moles of lithium. Finally, multiplying by the molar mass of lithium results in a mass of 20.0 grams. Thus, 20.0 grams of lithium will produce 35.25 liters of hydrogen.

For gaseous chemical reactions that occur at STP — standard temperature and pressure — the molar volume, 22.4 liters, is a constant. The conversion factor is used in stoichiometric calculations involving gases at STP.

Take, for example, the formation of water at STP. What volume of hydrogen is required to produce 2 grams of water?

Following the general conceptual plan, first, the mass of water is divided by its molar mass, to give the moles of water. Then the stoichiometric ratio is used to determine the moles of hydrogen. Finally, the mole to volume conversion factor at STP is used to get 2.5 liters of hydrogen gas.

Thus, 2.5 liters of hydrogen will produce 2 grams of water at STP.