## Amount of Substance

## Empirical Formula

**The simplest ratio of atoms of an element in a molecule**

## Molecular Formula

**The actual number of atoms of an element in a molecule**

## Moles

Measurement for the amount of atoms

Moles = Mass Ar or Mr |
Moles = concentration (M or mol/dm^3) x volume (dm^3) |

Concentration is also referred to as

**molarity**

**Water**has**density**of**1 g/cm^3***This means***1g**of water is**1 cm^3**## Finding Empirical Formula

Divide the mass by the atomic mass to give the amount of moles of each substance. Then find the ratio of each element

A simple method is to use a table:

Mass =Ar =Moles =Ratio = |
Element |

*If given as a percentage assume it is in grams*

To find the molecular formula the

**molecular mass**is needed and the empirical formula. By comparing the two you can discover the molecular formula

## Avogadro's Constant

This is the number of atoms in 12g of Carbon-12

## Stoichiometry

Discovering mass of other reactants or products in a reaction

b

If molar ratio is the same then the number of moles is the same

If different then it's:

b

If molar ratio is the same then the number of moles is the same

If different then it's:

**Known No. moles x**__unknown ratio__= unknown no. moles**known ratio**E.g.

Balanced ratio of moles between CuO and H2O so here are the same number of moles

Unbalanced ratio between Fe and Fe2O3 meaning the above calculation is required to work out the number of moles of Fe2O3

**CuO + H2 → Cu + H2O**Balanced ratio of moles between CuO and H2O so here are the same number of moles

**2Fe + 3O2 → Fe2O3**Unbalanced ratio between Fe and Fe2O3 meaning the above calculation is required to work out the number of moles of Fe2O3

**Molar Ratio does**__not__impact the Mr in any other calculation## Percentage Purity & Atom Economy

When working out how much of a molecule is pure then use:

__mass pure__x 100 = Percentage Purity**mass impure**When working out how much of the reactants is the reactant required use:

__mass of desired product__x 100**mass of reactants**## Ideal Gas Equation

**pV = nRT**

**p = Pressure (Pa)**

**V = Volume (**

__m^3__)**n = No. moles**

**R = Universal Gas Constant → 8.31**

**T = Temperature (K)**

**You must be able to get to these units from the units given like from decimetres cubed to metres cubed**