Chemical formulae equations and calculations

• Word equation


  A chemical reaction written using the names of the reactants and products
• Balanced chemical equation


  A chemical equation where the number of atoms of each element is the same on both sides of the equation
• State symbols


  • (g) means gas
  • (s) means solid
  • (l) means liquid
  • (aq) means aqueous
• Word equation


  • hydrochloric acid + sodium hydroxide -> sodium chloride + water
• Balanced chemical equation


  • HCl + NaOH -> NaCl + H2O
• Balancing an equation


  1. Make sure there are the same number of each element on each side of the equation

  2. Use big numbers at the front of a compound to balance it e.g. 3H2O
• Relative formula mass (Mr)


  Sum of the relative atomic masses of the atoms in the numbers shown in the formula
• In a balanced chemical equation, the sum of Mr of reactants in quantities shown = sum of Mr of products in quantities shown
• Mole (mol)


  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
• One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance
• Converting between moles and grams

  1. Moles = Mass / Relative atomic mass

  2. Mass = Moles x Relative atomic mass
• Calculating moles


  • Moles = 42g / 12 = 3.5 moles
• Interpreting chemical equations in terms of moles

  1. Find moles of one substance

  2. Use balancing numbers to find moles of desired reactant or product

  3. Mass = Moles x Molar mass (of the product)
• Percentage yield


  Amount of product produced / Maximum amount of product possible x 100
• Reasons percentage yield is less than 100%

  • Reaction may not go to completion because it is reversible
  • Some of the product may be lost when it is separated from the reaction mixture
  • Some of the reactants may react in ways different to the expected reaction
• Determining formula of a compound experimentally

  1. Weigh some pure magnesium

  2. Heat magnesium to burning in a crucible to form magnesium oxide

  3. Weigh the mass of the magnesium oxide

  4. Calculate the ratio of moles of magnesium to moles of oxygen
• Reasons why a reaction may not go to completion

  1. Reaction may not go to completion because it is reversible

  2. Some of the product may be lost when it is separated from the reaction mixture

  3. Some of the reactants may react in ways different to the expected reaction
• Yield


  The amount of product obtained
• Experiment to find formula of magnesium oxide

  1. Weigh some pure magnesium

  2. Heat magnesium to burning in a crucible to form magnesium oxide

  3. Weigh the mass of the magnesium oxide

  4. Calculate ratio of moles of magnesium to moles of oxygen

  5. Use ratio to form empirical formula
• Empirical formula


  The simplest whole number ratio of atoms of each element in a compound
• Molecular formula


  The number of atoms of each element in a compound
• Calculating empirical formula from molecular formula

  1. If there is a common multiple, the empirical formula is the simplest whole number ratio

  2. If there is no common multiple, the empirical formula is the same as the molecular formula
• Calculating molecular formula from empirical formula and relative molecular mass

  1. Find relative molecular mass of the empirical formula

  2. Divide relative molecular mass of compound by that of the empirical formula

  3. Multiply the number of each type of atom in the empirical formula by this number
• Concentration of a solution

  Can be measured in mass per given volume of solution e.g. grams per dm3 (g/dm3)
• Calculating concentration of a solution


  Concentration (g dm-3) = mass of solute (g) / volume (dm3)
• Calculating mass of solute in a given volume of a known concentration

  Mass = concentration x volume i.e. g = g/dm3 x dm3 (think about the units!)
• Molar volume of a gas

  • Equal amounts in mol. of gases occupy the same volume under the same conditions of temperature and pressure (e.g. RTP)
  • Volume of 1 mol. of any gas at RTP (room temperature and pressure: 20 degrees C and 1 atmosphere pressure) is 24 dm3
• Calculating volume of gas at RTP

  Volume (dm3) of gas at RTP = Mol. x 24
• Know how to determine the formula of a metal oxide by combustion (e.g. magnesium oxide) or by reduction (e.g. copper(II) oxide)