## Mole calculations

### Chemical formulae

There are numerous types of chemical formulae, each of which represent the proportion of atoms / ions of each element in a compound. Here we consider empirical and molecular formula. All ionic compounds are represented by empirical formula.

Empirical formula – simplest whole number ratio of atoms of each element present in the compound.

Molecular formula – exact number of atoms of each element present in the compound.

To work out the (empirical) formula for an ionic compound you first need to write each ion and its associated charge. The ions can be simple ions found by using the table below or polyatomic ions which are to be learnt off.

The formula is generated by ensuring that you multiply either (or sometimes both) ion by a number that allows the charges on the ions to be equal but opposite.

### Introduction to the Mole

A mole is a unit that refers to a specific amount of a chemical substance. One mole (1 mol) is the amount of a substance that contain particles (either atoms, molecules or formulae) of the substance. e.g. In 1 mol of water there arewater molecules, in 1 mol of helium there are atoms of helium (= Avogadro’s number).

RAM - Relative Atomic Mass (Ar) = the average mass of an atom compared to 1/12th the mass of an atom of carbon-12.

e.g. Chlorine mainly contains 2 isotopes: Cl-35 (75%) & Cl-37 (25%)

The Ar is an average of the two taking into account relative abundances:(35 x 0.) + 37 x 0.25)  = 35.5

RFM - Relative Formula Mass (Mr) = sum of RAMs of all atoms present in a formula

e.g. Mr of H2SO4: (1 x 2) + (32 x 1) + (16 x 4) = 98

Ar & Mr are the symbols of RAM and RFM respectfully.

The mass of one mole of atoms/molecules is the RAM/RFM expressed in grams.

#### Empirical formula calculations

To work out the empirical formula of a compound, you need to compare the number of moles of each element present in the compound. Therefore, to do this you first take the percentage or (mass in g) of each element present to work out the number of moles of each. Divide the number of moles of each element by the smallest value (number of moles) as this will will result in the simplest whole number ratio.

e.g.:

#### Concentration & Gas volumes

Not as clearly identified in the video is the method to quickly determine the volume of a gas when the number of moles is known. At r.t.p (room temperature & pressure, ~25C and 1 atm):  volume (dm3) =  mol x 24, and at s.t.p (standard temperature and pressure, = 0C and 1 atm): volume (dm3) =  mol x 22.4