SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Stoichiometry (Chemical Calculations) and the Mole Concept
DENSITY AND MOLAR MASS
Since the density of a gas is usually given in grams/liter of gas at STP, we can use the molar volume to molar mass relationship to solve the following types of problems.
EXAMPLE 1: Find the molar mass of a gas when the density is given as 1.25grams/liter.
Because it is known that 1 mole of a gas occupies 22.4 liters at STP, we can solve this problem by multiplying the mass of 1 liter by 22.4 liters/mole.
Even if the weight given is not for 1 liter, the same setup can be used.
REMEMBER For a dry gas at STP, the mass of 1 liter × 22.4 = the molar mass of the gas.
EXAMPLE 2: If 3.00 liters of a gas weighs 2.00 grams, find the molar mass.
You can also find the density of a gas if you know the molar mass. Since the molar mass occupies 22.4 liters at STP, dividing the molar mass by 22.4 liters will give you the mass per liter, or the density.
EXAMPLE 3: Find the density of oxygen at STP.
Oxygen is diatomic in its molecular structure.
O2 = molar mass of 2 × 16.0 or 32.0 g/mol 32.0 g/mol occupies 22.4 L.
You can find the density of a gas, then, by dividing its molar mass by 22.4 liters.
For a dry gas at STP,