(chemical calculations)

• elements in periods are often arranged from smallest to greatest relative atomic mass from left to right
• 1 mole always has 6.02x10^23 (avrogados' number) atoms/particles
• if the mass of a compound is already known, add the masses together for the Mr
• if the mass of a compound is not known, divide mass/mol (m/n = Mr)
• percentage yield = (actual yield/theoretical yield)x100
• actual yield = amount of substance obtained in an experiment
• theoretical yield = amount of substance obtained from equation
• percentage of substance in compound = (substance mass/compound mass)x100
• molecular formula tells the exact number of atoms in a molecule
• empirical formula tells the ratio of atoms in a substance (simplest form)
• practical method to determine empirical formula:

  1. use top pan balance to measure mass of crucible and Mg ribbon
  2. coil Mg ribbon
  3. place crucible on pipe clay triangle, on tripod, on heatproof mat
  4. heat with bunsen burner on high heat
  5. from time to time, lift the lid to allow Mg ribbon to react with oxygen
  6. after reaction is finished, cool down crucible
  7. measure mass of crucible + lid + contents
• when ionic compounds crystallise from solution they absorb water into structure (different compounds absorb different ratios of water)
• method of water crstallisation:

  1. measure the mass of an evaporating basin
  2. add 3g solid
  3. heat the salt until reaction completes (stir)
  4. measure change in mass + calculate