Quantitative Chemistry
It is possible to calculate:
The percentage of an element that is in a compound given the relative mass of the element in the formula and the relative formula mass of the compound
The empirical formula of a compound given the masses or percentages of the elements in the compound
The masses of reactants and products in a reaction given the balanced symbol equation of the reaction.
No atoms are gained or lost during a chemical reaction. However, it is not always possible to obtain the calculated amount of a product in a reaction because:
If the reaction is reversible, it might not go to completion
Some of the product might be lost when it is separated from the reaction mixture
Some of the reactants might react in different ways from what was predicted.
The amount of a product obtained from a chemical reaction is called the yield. When the actual yield is compared with the theoretical yield (as calculated using the masses or formulae of the reactants), it is called the percentage yield.
In some reactions, the products can react to form the original reactants. These are called reversible reactions. They are represented:
A + B ⇌ C + D
For example: ammonium chloride ⇌ ammonia + hydrogen chloride