Calculate the density of oxygen, O_{2}, under each of the following conditions:

• STP

• 1.00 atm and 30.0 °C

Express your answers numerically in grams per liter. Enter the density at STP first and separate your answers by a comma.

Pressure and temperature affect the amount of space between gas molecules, which affects the volume and, therefore, the density of the gas since

$\mathbf{density}\mathbf{=}\frac{\mathbf{mass}}{\mathbf{volume}}$

The molar mass of a substance, however, is a constant and can be used to identify an unknown gas sample. Molar mass is found by dividing the mass of a sample (in grams) by the number of moles in that sample. The number of moles of gas can be calculated using the ideal gas law

$\mathbf{PV}\mathbf{=}\mathbf{nRT}$

which can be rearranged as

$\mathbf{n}\mathbf{=}\frac{\mathbf{PV}}{\mathbf{RT}}$

Given the number of moles of a gas and its molar mass, you can calculate the mass of the gas. Since density is equal to the ratio of the mass and volume, you can then divide by the volume to find density.

Alternatively, you can use the ratio n/V from the ideal gas equation where *n* is the number of moles and *V* is the volume, and convert from moles per unit volume to grams per unit volume using molar mass.

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