# The Mole Concept

#### The Mole Concept Understanding

- The
**mole concept**is a convenient method of expressing the amount of a substance. Any substance weighs a certain amount, and this weight depends on what the material is made of. - In chemistry, the ‘amount’ of a substance we have is often measured in
**moles**. - One
**mole**of any substance contains approximately**6.02 x 10^23**entities of the substance. This is known as**Avogadro’s number**.

#### Relative Atomic and Molecular Mass

- The
**relative atomic mass**(Ar) of an element is the mass of an atom of an element compared with one twelfth of the mass of an atom of carbon-12. - The
**relative molecular mass**(Mr) of a substance is the sum of the relative atomic masses of the atoms in a molecule. - The mass in grams of one mole of a substance is equal to the relative molecular mass for that substance.

#### Moles and Gases

- At room temperature and pressure (rtp), one mole of any gas occupies a volume of
**24.0 dm^3**. - The volume that one mole of gas occupies at any temperature and pressure can be determined using the
**ideal gas equation**.

#### Moles in Solution

- The concentration of a solution in
**mol dm^−3**can be calculated using the formula: concentration (mol dm^−3) = number of moles/volume (dm^3). - We can calculate the number of moles in a known volume of a solution of known concentration.
- We can also calculate the volume of a solution of known concentration that is needed to provide a certain number of moles of solute.

#### Reacting Masses and Ratios

- A balanced chemical equation tells us the ratio in which the reactants react together and form the products.
- From this, we can calculate the
**masses**of reactants used and products formed in a reaction. - The
**stoichiometric coefficients**(the numbers in front of the symbols/formulas in a balanced chemical equation) give us the**mole ratio**of reactants and products. - Knowing the mole ratio, we can find out
**how much of each reactant is needed**, or**how much of each product will be produced**.

#### Calculations Involving Moles

- Calculations often involve working out the number of moles in a certain mass of a substance.
- The number of moles in a given mass of a substance can be calculated using the formula:
**number of moles = mass (g) / Mr**. - Similarly, the mass of a certain number of moles of a substance can be calculated using the formula:
**mass (g) = number of moles x Mr**.