Chemical formulae, equations and calculations.

• Atomic number = Number of protons
• Isotope

  Atoms of the same element that have the same number of protons but a different number of electrons.
• Relative atomic mass (Ar)

  Is the average mass of all the isotopes of an element compared with the mass of one atom of carbon-12.
• Mole


  Is the amount (or mass) of a substance that contains 6x10^23 particles or atoms or molecules.
• Avogadro's number


  Is the number of particles or atoms or molecules in 1 mole of substance.
• Relative formula mass (Mr)


  Is the sum of the atomic masses of all the atoms represented in the chemical formula. The relative formula mass of a substance, in grams, is known as one male of that substance.
• Molar mass


  Is the mass (or amount) of one mole of a substance. It is normally expressed in units of g/mol.
• Empirical formula


  The ratio of atoms in their lowest terms (i.e. smallest numbers). The simplest formula.
• Molecular Formula


  The number of atoms present in one molecule of a given substance.
• Hydrated


  This refers to a crystalline compound containing water molecules.
• Anhydrous


  This refers to a substance that contains no water molecules.
• Water of crystallisation

  This refers to water molecules that form an essential part of the crystalline structure of the compound.
• Percentage yield


  This compares the amount of product that is actually produced in a reaction with the maximum amount that it could possibly produce.
• Equation for calculating relative atomic mass (RAM)


  ((% of isotope 1 x mass of isotope 1)+(% of isotope 2 x mass of isotope 2)) /100
• Equation to find mass using moles and Mr

  Mass = Mr x mol
• Finding the empirical formula

  STEP 1

  Find separate weight or %

  
  

  STEP 2

  Turn into moles (divide by relative atomic mass)

  
  

  STEP 3

  Divide by the smallest to get a whole number.
• Calculating molecular formula

  STEP 1

  Find the empirical formula.

  
  

  STEP 2

  Find the relative formula mass (Mr) of the empirical formula.

  
  

  STEP3

  Divide the relative formula mass (Mr) of the compound by the Mr of the empirical formula.

  
  

  STEP 4

  Multiply each element in the empirical formula by the number obtained in step 3 to find the molecular formula.
• Carrying out a practical to find the formula of magnesium oxide

  1. Weigh a clean, dry crucible and lid.
  2. Clean a 10 cm strip of magnesium with emery paper. Place the magnesium in the crucible and weigh the crucible, lid and magnesium.
  3. Set up your Bunsen burner, tripod and clay triangle.
  4. Heat the crucible strongly and from time to time, carefully lift the crucible lid a small distance and then replace it, but try not to lose any white smoke.
• Carrying out a practical to find the formula of magnesium oxide (pt 2)

  1. When the reaction has finished, allow the crucible and lid to cool before weighing the crucible, lid and magnesium oxide again.
  2. Record all of the weights measured and calculate the mass of magnesium and oxygen, before finding the formula.
• Experiment to find the formula of hydrated copper(II) sulfate

  1. Find the mass of your crucible
  2. Place 2-3 spatulas of blue copper(II) sulfate in the crucible and weigh it again.
  3. Heat the crucible with a bunsen burner until the blue powder has turned completely white, but do not heat it so much that it turns black.
  4. Allow the crucible to cool before reweighing.
  5. Use your results to find the formula.