Quantitative Chemistry

When an experiment is done that gives masses that have reacted to form a compound, it may be necessary to work out the empirical, (simplest possible) formula of the product. This is done as follows:

The relative amount of each substance is found by dividing the reacting masses by the relative atomic / molecular mass. The simplest ratio between these numbers is then found, and this ratio put into the formula. This is best done in the form of a table, as follows.

Example

A compound is found to be, by mass, 20.14% iron, 11.53% sulphur, 23.01% oxygen and 45.32% water. What is the empirical formula?

Element

Fe

S

O

H2O

% mass

20.14

11.53

23.01

45.32

Relative amount

0.360

0.360

1.438

2.52

Ratio

1

1

4

7

Therefore the empirical formula is FeSO4.7H2O. This suggests that the compound is hydrated iron sulphate.

When writing equations, it is important to know the oxidation states of all the atoms in the compound, so that they can be arranged in the right proportions. When writing ionic equations, it is important to remember that, usually, the part of the equation that is wanted is only that in which there is some change. This is often a change of state caused by precipitation - such as in the following reaction:

For this reaction, if asked to write an ionic equation for the precipitation, it would be:

Note that it is vitally important to include the state symbols in ionic equations.

The Avogadro constant is the number of atoms in 12g exactly of carbon-12, and is roughly equal to 6.02x1023. The Avogadro number of anything is called one mole of that species, and is most often used with atoms and molecules. The following formulae are used when working with amount of substance:

Where all volumes are in cm3.

The chemical equation gives the makeup of the compounds involved and the ratio of the moles of the reactants and products reacting. This allows us to work out the reacting masses, or vice versa, if the masses data is given.