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

Relative Atomic Mass and Relative Molecular Mass

The isotope ${\ce {^{12}C}}$ is the standard for relative mass. A relative atomic mass of 1 is 1 twelfth of the mass of 1 atom of ${\ce {^{12}C}}$. Carbon is used due to its relative abundance in nature.

The Mole and the Avogadro Constant (L)

Avogadro Constant and The Mole

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

$L=6.023\times 10^{23}$

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

$1\ mole\ of\ particles=6.023\times 10^{23}\ particles$

The mass of 1 mole of particles of a substance with a relative atomic mass of 1, is precisely 1g.
1 mole of ${\ce {^{12}C}}$ (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 clearer.

Isotopes

Isotopes have different mass numbers but the same atomic numbers. They are particles with the same number of protons (same element) but with different number of neutrons. ${\ce {^{14}C}}$ is an isotope of ${\ce {^{12}C}}$ as they both have 6 protons but different number 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

Definitions:

• Empirical formula: The simplest whole number ratio of atom types present in a molecule of a compound.
• Molecular formula: 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.