Molar Volume of GasSection 4.3

Pg. 169-171

This often involves integrating two or more concepts. I.e. Boyle’s and Charles’ Laws were combined to create the

Combined Gas Law

Similarly, Avogadro’s Theory and the Mole concept can also be combined: Avogadro: “Equal volumes of any gases at the same temperature

and pressure contain equal numbers of entities”

Therefore, for all gases at a specific temperature and pressure, there must be a certain volume, the molar volume, that contains one mole of entities

Molar Volume: the volume that one mole of a gas occupies at a specified temperature and pressure

The Evolution of Scientific Knowledge

Molar volume is the same for all gases at the same temperature and pressure (remember, all gases have the same physical properties) At STP, molar volume = 22.4 L/mol (101.325 kPa and 0°C)

At SATP, molar volume = 24.8 L/mol (100 kPa and 25°C)

This can be used as a conversion factor just like molar mass!

Molar Volume

At STP, one mole of gas has a volume of 22.4 L, which is approximately the volume of 11 “empty” 2 L pop bottles.

STP = 22.4L/molSATP = 24.8 L/mol

Chemists created the concept of molar volume to convert between volume and chemical amount

Molar Volume as a Conversion Factor

V nlitres mol

x 1 mol

x L

x x L 1 mol

Remember the conversion factor will be different at STP and SATP!

Why are we dealing with molar volume instead of molar mass??? It’s a lot easier to measure the volume of a gas than trying to

measure its mass.

You would have to trap the gas in a container and measure its mass on a balance and them make corrections for the buoyant force of the surrounding air . . . not easy

Molar Volume

Molar Volume – Practice

1. Calculate the volume occupied by 0.024 mol of carbon dioxide at SATP.

VCO2 : 0.024 mol x ( 24.8 L) = 0.60 L 1 mol

2. What chemical amount of oxygen is available for a combustion reaction in a volume of 5.6 L at STP?

STP = 22.4L/molSATP = 24.8 L/mol

nO2 : 5.6 L x ( 1 mol ) = 0.25 mol 22.4 L

Molar Volume – Practice

3. What volume does 3.50 g of helium gas (He) occupy at SATP?

nHe : 3.50 g x ( 1 mol ) = 0.875 mol 4.00 g

VHe : 0.875 mol x ( 24.8 L) = 21.7 L 1 mol

Once these calculations are clearly understood, they can be combined into a single calculation using unit analysis. All units except the final unit will cancel.

STP = 22.4L/molSATP = 24.8 L/mol

VHe : 3.50 g x ( 1 mol ) x ( 24.8 L) = 21.7 L 4.00 g 1 mol

Molar Volume – Practice

4. A propane tank for a barbecue contains liquefied propane. IF the tank mass drops by 9.1 kg after a month’s use, what volume of propane gas at SATP was used for cooking?

Molar mass (M): C3H8(g) = 44.11 g/mol

V C3

H8(g) : 9.1 kg x (1 mol ) x ( 24.8 L) = 5.1 kL

44.11g 1 mol

What if I wanted your answer in litres?

STP = 22.4L/molSATP = 24.8 L/mol

5.1 kL x 1000L = 5100 L = 5.1 x 103 L 1 kL

Summary

STP = 22.4L/molSATP = 24.8 L/mol

Molar volume: the volume that one mole of a gas occupies at a specified temperature and pressure

Homework: pg. 171 #5 - 12