Chemistry Question

Question

Dry air (0% humidity) is approximately   and  by moles. Calculate the molar mass and density of dry air at 0 °C and 1.0 atm. Assume ideal behavior, and use two significant figures in the answers.

Molar mass = ___ g/mol

Density=___g/L

Typical outdoor air also has water vapor,  mixed in with the other gases. Consider the molar mass and density of humid air compared to dry air.

question: The molar mass of humid air is (less than / equal to / greater than) the molar mass of dry air.

question: The density of humid air is (less than / equal to / greater than) the density of dry air.

Answer 1

PV = nRT ... Ideal Gas Law

Since the temperature is 0ºC (273K) and the pressure is 1.0 atm, this is STP (standard temperature/pressure)

Thus 1 mole of gas (air) = 22.4 L

Density is mass/volume. We can assume 22.4 L and 1 mole of air. Now, we find the mass of 1 mole of air.

78% N₂ = 0.78 x 28 g/mol = 21.8 g N2

21% O₂ = 0.21 x 32 g/mol = 6.72 g O2

1% Ar = 0.01 x 39.95 g/mol = 0.40 g Ar

Total mass = 28.9 g

Molar mass = 28.9 g/mole

Density = 28.9 g/22.4 L = 1.29 g/L