2.1

• Amount of substance is the name given when counting the number of particles in a substance
• Amount of substance is often seen in calculations using the letter/symbol n
• The units for amount of substance are moles/mol
• Amount of substance links to the Avogadro constant, N, which is the number of particles equivalent to the relative atomic, molecular or formula mass of a substance
• The Avogadro constant applies to atoms, molecules, ions and electrons
• The value of N is 6.02 x 10^23 g mol^-1
• The mass of a substance with this number of particles is called a mole (mol)
• Molar mass is the mass of a substance that contains the same number of fundamental units as exactly 12.00 g of carbon-12
• The amount/number of moles of a substance, n, the mass of the substance, m, and the molar mass, M, are linked by the equation: n = mass / molar mass
• The molar gas volume is the volume occupied by one mole of any gas, at room temperature and pressure
• The molar volume is equal to 24 dm^3
• One mole of any element is equal to the relative atomic mass of the element, in grams
• If you had one mole of carbon in your hand, you would be holding 6.02 x 10^23 atoms of carbon which have a mass of 12.0 g
• If you were holding one mole of water, you would be holding 6.02 x 10^23 molecules of water which have a mass of 18.0 g (2 hydrogen + 1 oxygen = (2 x 1.0) + 16.0 = 18.0 g)
• Determining Formulae
• The molecular formula shows the number and type of each atom in a molecule
• The empirical formula shows the simplest whole number ratio of the elements present in one molecule of the compound
• Empirical formula: simplest whole number ratio of a compound
• Empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound
• Empirical formula is calculated from knowledge of the ratio of masses of each element in the compound
• Molecular formula
• The molecular formula gives the exact numbers of atoms of each element present in the formula of the compound
• The molecular formula can be found by dividing the relative molecular mass of the molecular formula by the relative formula mass of the empirical formula
• Hydrated salts & Water of Crystallisation
• Water of crystallisation is when some compounds can form crystals which have water as part of their structure
• A compound that contains water of crystallisation is called a hydrated compound
• The water of crystallisation is separated from the main formula by a dot when writing the chemical formula of hydrated compounds
• A compound can be hydrated to different degrees
• The conversion of anhydrous compounds to hydrated compounds is reversible by heating the hydrated salt
• Reaction Calculations
• The number of moles of a substance can be found by using the equation: number of moles (mol) = mass of a substance (g) / molar mass (g mol^-1)
• To calculate reacting masses, the balanced chemical equation is required
• The balanced chemical equation shows the ratio of moles of all the reactants and products, also known as stoichiometry
• To find the mass of products formed in a reaction, the following pieces of information are needed:

  • The mass of the reactants
  • The molar mass of the reactants
  • The balanced equation
• Calculate the mass of magnesium oxide that can be made by completely burning 6 g of magnesium in oxygen:

  • Balanced symbol equation: 2Mg (s) + O2 (g) → 2MgO (s)
  • Calculate moles of magnesium used: 0.25 moles
  • Find the mass of magnesium oxide: 10 g
• The stoichiometry of a reaction can be found if the exact amounts of reactants and products formed are known
• The amounts can be found by using the equation: number of moles (mol) = mass of a substance (g) / molar mass (g mol^-1)
• In the combustion of 50 cm^3 of propane reacting with 250 cm^3 of oxygen, 150 cm^3 of carbon dioxide is formed suggesting that the ratio of propane : oxygen : carbon dioxide is 1 : 5 : 3:

  • Balanced symbol equation: C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (l)
• The concentration of a solution is the amount of solute dissolved in a solvent to make 1 dm^3 of solution
• To calculate the mass of a substance present in a solution of known concentration and volume:

  • Rearrange the concentration equation: number of moles (mol) = concentration (mol dm^-3) x volume (dm^3)
  • Multiply the moles of solute by its molar mass: mass of solute (g) = number of moles (mol) x molar mass (g mol^-1)