# A-level Chemistry/AQA/Module 1/Amount of Substance

## Relative Atomic Mass and Relative Molecular Mass

The isotope 12C is the standard for relative mass: a relative atomic mass of 1 is 1 twelfth of the mass of 1 atom of 12C.

## The Mole and the Avogadro Constant (L)

### Avogadro Constant and The Mole

The Avogadro Constant is a quantity and is given the symbol L:

```   L = 6.023 x 1023
```

A mole of particles is L particles. I.e.

```   1 mole of particles = 6.023 x 1023 particles.
```

The mass of 1 mole of particles of a substance with a relative atomic mass of 1, is precisely 1g.
1 mole of 12C (relative atomic mass 12) is 12g.

### Molarity

is a quantitative expression of the concentration of a solution.

Or more simply, it is the concentration when measured in moles per dm3.

Sometimes you will see 'm' used as a short version of this. E.g. 5m acid means 5 moles per cubic decimeter. A capital M should not be used as that means meters so a small m should be used or mol to be extra clear.

### Isotopes

Isotopes have different mass numbers but the same atomic numbers; in other words they have the same number of protons but different numbers of neutrons.

## The Ideal Gas Equation

$pV=nRT$

p is the pressure of the gas in Pa (Nm-2)

V is the volume of the gas in m3

n is the number of moles

R is the gas constant (8.31 J K-1 mol-1)

T is the temperature in Kelvin (equivalent to degrees Celsius + 273)

## Empirical and Molecular Formulae

An empirical formula is the simplest whole number ratio of atom types present in a molecule of a compound.
Molecular formula is the actual number of atoms of each element present in a molecule of a compound.
To determine a molecular formula you need the given mass of the substance and the empirical formula mass. Divide the given mass by the empirical formula mass.
Multiply the given answer by the ratios in the empirical formula and you will have the molecular formula.