Solutions & Gases: Concentration and the Ideal Gas Equation (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about concentration and the ideal gas equation.

The two are covered together because each one is a way of turning a volume you can measure into a number of moles you cannot.

Concentration tells you how much solute is dissolved in a solution. The ideal gas equation relates the pressure, volume and temperature of a gas to the number of moles it contains. Both of them start from a volume and end at an amount in moles, which is why they sit together.

We start with the concentration of solutions, and the equation linking moles, volume and concentration. Then gases: the volume that one mole of any gas occupies at room temperature and pressure. After that, what makes a gas ideal, and finally the ideal gas equation itself.

The concentration of a solution is the amount of solute dissolved in a solvent to make one decimetre cubed of solution. The solute is the substance that dissolves, and the solvent is often water.

Concentration in moles per decimetre cubed is the number of moles of solute divided by the volume of the solution in decimetres cubed. A concentrated solution has a large amount of solute, and a dilute solution has a low one.

Rearranged, that same equation gives you the number of moles: concentration multiplied by volume. Multiply those moles by the molar mass and you have the mass of solute present. The molar mass has the same numerical value as the relative atomic or relative formula mass. Two conversions come first: mass in grams goes to moles, and volumes in centimetres cubed go to decimetres cubed.

This is the first of the two routes from a measured volume to an amount in moles.

Avogadro's hypothesis states that equal volumes of gases contain the same number of molecules.

That gives you the molar gas volume. At room temperature and pressure, one mole of any gas occupies 24.0 decimetres cubed. Room temperature here is 20 degrees Celsius, and room pressure is one atmosphere.

The volume of a gas in decimetres cubed is the amount of gas in moles multiplied by 24, and the amount of gas in moles is the volume divided by 24. It is true for any gas.

This is the same route as concentration, applied to a gas instead of a solution: a volume goes in, an amount in moles comes out.

The kinetic theory of gases describes the behaviour of gas particles, and it forms the basis of the ideal gas model.

Gas particles move rapidly and randomly in straight lines. Their own volume is negligible compared with the volume of the container. There are no intermolecular forces between them. Collisions between particles, and between particles and the container walls, are perfectly elastic, so no kinetic energy is lost. The average kinetic energy of the particles is directly proportional to the absolute temperature. A gas that obeys all of those assumptions is an ideal gas.

Heat a gas at constant pressure and the particles gain kinetic energy, colliding more frequently and with greater force against the walls. To keep the pressure constant the particles must move further apart, so the volume increases. At constant pressure, the volume of a gas is directly proportional to its absolute temperature in kelvin.

Those assumptions are what allow a single equation to take a gas volume to a number of moles.

The ideal gas equation is PV equals nRT. It shows the relationship between pressure, volume, temperature and the number of moles of an ideal gas.

Pressure is in pascals, volume in metres cubed, temperature in kelvin, and R is the gas constant, 8.31 joules per kelvin per mole. Rearranged, it gives you the number of moles from a pressure, a volume and a temperature. This is the second route from a volume to an amount in moles, and unlike the 24.0 figure it is not tied to room temperature and pressure.

Real gases do not behave ideally at very low temperatures and high pressures. Under those conditions the particles are much closer together, so intermolecular forces become significant. These attractions pull particles slightly away from the container walls, reducing the force of collisions, and the measured pressure comes out lower than the ideal gas model predicts. At high pressures the volume of the particles themselves is no longer negligible either.

Temperature in this equation is in kelvin. Add 273 to the Celsius temperature, so 100 degrees Celsius is 373 kelvin.

The units are important throughout: know what units each quantity should be in, and do the conversions before you calculate. Pay particular attention to the unit for volume which should be metres cubed. You also need to be able to rearrange the ideal gas equation to work out any part of it.

Concentration is the number of moles of solute per decimetre cubed of solution.

One mole of any gas occupies 24.0 decimetres cubed at room temperature and pressure.

The ideal gas equation, PV equals nRT, does the same job without being tied to room conditions, with pressure in pascals, volume in metres cubed and temperature in kelvin.

All of them take a volume you can measure and give you an amount in moles.