We can represent a chemical change by a chemical formula.
Chemicals formulas have the following syntax:
reactants -(chemical reaction)> new product All chemicals on the right hand side of a formula are called reactants and on the left hand side are called products.
2 O2 + 2 H2 -> 2H2O
The above is an example of a chemical formula. However, the chemical formula is not possible in it's current state. In order for a chemical formula to make sense, we first must balance it.
O2 + 2 H2 -> 2H2O
Balancing involves taking making sure the number of atoms on one side of the forumla match the amount on the other side. Unless your formula is part of a nuclear reaction, in which mass is converted into energy, the amount of mass of reactants must always equal the product.
So we know that the mass must always be the same on both sides of a formula, but we also know that different molecules have different molecular weights. From a chemical formula, it is then possible to discovered exactly how much a certain molecule (usally in moles) of one reactant versus another is required. For example the above formula, it can be discovered that for every 1 mole of oxygen, 2 moles of hydrogen are required for every two moles of water.
You can use these "molar ratios" to find alot of informaton about how to prepare the reaction, namely how much mass of each reactant is required for a effiecent reaction to occur.