Edexcel Chemistry: Formulae, Equations & Calculations

• These notes cover Edexcel IGCSE Chemistry, specifically Topic 1: Principles of Chemistry, focusing on chemical formulae, equations, and calculations.

Chemical Equations

• Word and balanced chemical equations, including state symbols, are used to represent reactions.
• State symbols: (g) for gas, (s) for solid, (l) for liquid, (aq) for aqueous solutions.
• A word equation example: hydrochloric acid + sodium hydroxide -> sodium chloride + water.
• A balanced chemical equation example is HCl + NaOH -> NaCl + H₂O.
• Balancing equations involves making sure the number of each element is the same on both sides.

Relative Formula Masses

• Relative formula mass (Mr) is the sum of the relative atomic masses of the atoms in the formula.
• In balanced chemical equations, the sum of Mr of reactants in quantities shown equals the sum of Mr of products in quantities shown.

Moles

• The mole (mol) is the unit for the amount of a substance.
• The mass of one mole of a substance in grams is numerically equal to its relative formula mass.
• The Ar of Iron is 56, therefore, one mole of iron weighs 56g.
• The Mr of nitrogen gas (N₂) is 28 (2x14), so one mole is 28g.
• One mole of a substance contains the same number of the stated particles as one mole of any other substance.

Calculations Involving Moles

• Moles, mass, and relative atomic mass or relative formula mass can be interconverted:
  • moles = mass / relative atomic mass
  • mass = moles x relative atomic mass

Reacting Masses

• Chemical equations can be interpreted in terms of moles.
• Mg + 2HCl →MgCl₂ + H₂ represents 1 mol Magnesium reacts with 2 mol Hydrochloric acid to produce 1 mol Magnesium chloride and 1 mol Hydrogen.
• Masses of reactants and products can be calculated.
  • Find moles of one substance: moles = mass / molar mass.
  • Use balancing numbers to find the moles of the desired reactant or product.
  • Mass = moles x molar mass of the product.

Percentage Yield

• Percentage yield is calculated using the formula: Percentage yield = (Amount of product produced / Maximum amount of product possible) x 100.
• A reaction may be reversible, some product may be lost, or reactants may react in unexpected ways.
• The "amount of product obtained" is known as the yield.

Determining Empirical Formulae

• The formula of magnesium oxide can be determined through an experiment:
  • Weigh pure magnesium.
  • Heat magnesium in a crucible to form magnesium oxide.
  • Weigh the mass of the magnesium oxide.
• Quantities known: mass of magnesium used and mass of magnesium oxide produced
• Calculations:
  • mass oxygen = mass magnesium oxide - mass magnesium
  • moles magnesium = mass magnesium / molar mass magnesium
  • moles oxygen = mass oxygen / molar mass oxygen
  • Calculate the ratio of moles of magnesium to moles of oxygen and use the ratio to form the empirical formula.

Empirical and Molecular Formulae

• Molecular formula indicates the actual number of atoms of each element in a compound.
• Empirical formula represents the simplest whole number ratio of atoms.

Calculating Empirical and Molecular Formulae

• From a molecular formula with a common multiple e.g. Fe₂O₄, the empirical formula would be FeO₂.
• If there is no common multiple, the empirical and molecular formulae are the same.
• Find relative molecular mass of the empirical formula, divide the relative molecular mass of the compound by that of the empirical formula, then multiply.

Calculations Involving Solutions

• Concentration of a solution can be measured in mass per given volume (g/dm³).
• concentration (g/dm³) = mass of solute (g) / volume (dm³).
• mass = conc x vol i.e. g = (g/dm³) x dm³.

Gas Volumes

• Equal amounts in mol. of gases occupy the same volume under the same conditions.
• Volume of 1 mol. of any gas at RTP is 24 dm³.
• The relevant equation to use is: Volume (dm³) of gas at RTP = Mol. x 24.

Determining Formulae of Metal Oxides

• The method of determining the formula of a metal oxide by combustion or reduction is similar to the experiment used to find formula of magnesium oxide.