# 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.