November 18th - More Molar

We are expanding our chart! Today, we added volume to it. In addition, we will be learning how to convert from moles to volume, vice versa, and even from volume to mass.

The volumes we are working with pertain to gases. At a specific pressure and temperature, one mole of any gas occupies the same volume. Therefore, the volume occupied by one mole of oxygen gas is equivalent to the volume occupied by one mole of hydrogen. However, the temperature and pressure of these two have to be the same.

Molar volume – to volume occupied by one mole of a substance

When temperature increases, the volume will increase. This is one of the laws of kinetics. It tells us that a particle vibrates and moves faster. This results in more space between particles and a greater volume.

The volume we will be measuring is performed at STP (standard temperature and pressure). This refers to 0^oC and 101.3 kPa. At STP, one mole equals 22.4L of a gas. We can conclude that:

22.4L is the molar volume at STP

Let’s try a few examples.

How many litres will there be in 5 mol of H₂ gas?

5.0 mol x (22.4L/1 mol) = 112L = 1.1 x 10²L

How many litres will there be in 5 mol of O₂ gas?

5.0 mol x (22.4L/1 mol) = 112L = 1.1 x 10²L

It`s exactly the same! This supports the fact that at a specific pressure and temperature, one mole of any gas occupies the same volume.

And remember to use ‘em significant digits!

Let’s try converting more volume to mass.

A box is 6.0m long, 4.0 meters wide, and 3.0 high. If it is filled with oxygen gas, determine the mass of O₂.

The steps to solve this are as follows:

1.    6.0 x 4.0 x 3.0 = 72m³

2.    72m³ x (1000L/m³) = 72000L

3.    72000L x (1 mol/22.4L) = 3000 mol

4.    3000 mol x (32.0g / 1 mol) = 96,000 g O₂ = 9.6 x 10³ g.

That’s about all we did, but let’s briefly explore the ideal gas law. This is the equation of state of a hypothetical ideal gas. It is given by the equation: PV = nRT, where P is pressure, V is volume, n is number of moles, R is the constant (8.3144621), and T is the temperature (K).

SATP is similar to STP, however, the temperature is 25^oC instead of 0^o. Also, we perform this at 100 kPa rather than 1 atm (101.3 kPa). It stands for Standard Ambient Temperature and Pressure.

At SATP, the volume occupied by one mole is 24.8L/mol. 

Posted by Andrew.