Higher and foundation tier
As a chemist you need to be able to calculate the masses of reactants you need to make a particular product 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 masses from equations we should consider 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 particles present in the reactants appear in the products. All that really
happens is the particles are rearranged as they go from reactants to products.
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.
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.
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?