Higher and foundation tier

As a chemist you need to be able to calculate the masses of the reactants you need to make a particular product during a chemical reaction and vice versa....the way to do this is by using the idea of moles. Remember the mole is a unit of measurement of mass in chemistry. It is the mass of 6x1023 particles. Before we can calculate reacting masses from equations we should consider the law of conservation of mass.

### The law of conservation of mass

If you think about what happens to the particles during a chemical reaction you would probably realise that all the particle present in the reactants appear in the products. All that really happens is the particles are rearranged as they go from reactants to products.

### For example- When hydrogen molecules react with oxygen to make hydrogen oxide (water).

If you study the image below, which simply shows hydrogen molecules reacting with oxygen molecules to make water, you will see that NO atoms appear or disappear. All the atoms present in the reactant appear in the products - just arranged in a different way! This is the law of conservation of mass in a nutshell- chemistry is not magic, you cannot make things appear and disappear. The total mass of all the particles in the reactants will be the same as the total mass of all the particle in the products.

### Law of conservation of mass

Consider the two reactions shown in the image below. In the first example, the contents of one beaker are poured into the other, the balance scale reads 250g before and after the reaction. This is probably what you expect considering what was said above, you cannot make atoms appear or disappear. Everything you start with you end up with- just a bit more mixed up perhaps!

However in the second example, chalk or calcium carbonate is added to hydrochloric acid and this time the balance reading seems to have gone down. The key to explaining why is in the state symbols for the reactants and products. In the first example all the reactants and products are either solutions - state symbol (aq) or solids -state symbol (s). Nothing enters or leaves the beakers. However in the second example one of the products is a gas- carbon dioxide. Carbon dioxide is a heavy gas, here it leaves the beaker and enters the atmosphere. This means that one of the products is escaping into the air, so the mass of the products will be less than the mass of the reactants.

### Do gases have mass?

Gases have mass, some people mistakenly believe that gases have no mass. CO2 has a Mr of 44, this means that 1 mole of carbon dioxide gas has a mass of 44g. It's a heavy gas, it is used in fire extinguishers because it is so heavy it surrounds the fire and prevents the air from getting to it.

### Gases have mass, despite what many student think!

If you assume that the air is 80% nitrogen (N2) and 20% oxygen (O2). The Ar of nitrogen is 28 and the Ar of oxygen is 32 then the average mass of the gases in the air is about 29. Carbon dioxide gas, CO2 has an Mr of 44, so it's heavier than air. That is why balloons filled with CO2 sink. A balloon filled with hydrogen gas rises rapidly because hydrogen gas, formula H2 has an Mr of 2. So 1 mole of hydrogen is 2g. The gases in the air as mentioned have an average mass of about 29. Can you explain why helium balloons, He, Mr=4 rise in air?

### Key points

• You cannot make particles appear and disappear. The total mass of the reactants and products in a chemical reaction will always be the same.
• If the mass appears to be going down during a chemical reaction then it probably because a gas is being released into the air.
• magnesium metal burns in air to form a white powder called magnesium oxide, an equation for the reaction is shown below:
##### magnesium + oxygen → magnesium oxide
Here the metal magnesium is having a gas, oxygen added to it, so what do you think will happen to its mass this time? Well as was mentioned above gases have mass, so adding oxygen gas to a piece of magnesium will increase its mass.