since both of the substances in question are not gases. Additionally, the
relationship 22.4 L = 1 mole of the gas cannot be used since the reaction is not
taking place at STP. To solve problems such as these, the Ideal Gas Law, PV =
nRT, must be considered. Recall from Chapter 5 that the Ideal Gas Law can be
manipulated to solve for the moles of gas at any temperature and pressure as long
as the volume is supplied ( ). Likewise, the volume of a gas can be
determined if the number of moles of the gas is known along with its temperature
and pressure ( ).
Example 2
Based on the reaction below, what volume of oxygen is produced from the
decomposition of 5.00 g of hydrogen peroxide if the oxygen produced was
collected at 70.0°C and 1.25 atm pressure?
2H 2 O 2 (aq) → 2H 2 O(l) + O 2 (g)To solve this problem, dimensional analysis should be used to determine the
number of moles of O 2 that would be produced:
The number of moles of O 2 produced can now be plugged into the Ideal GasLaw to find the volume of that amount of O 2 at the temperature and pressure
outlined in the problem:
TIP
The paragraph describes a practical example of a “limiting reactant.”PROBLEMS WITH AN EXCESS OF ONE REACTANT OR A
LIMITING REACTANT
It will not always be true that the amounts given in a particular problem are
exactly in the proportion required for the reaction to use up all of the reactants. In