Amount of substance and mole

• One mole of a substance contains $6.02\times10^{23}$particles. This is called the Avogadro constant Na.
• Molar mass is the mass per mole of a substance. The units of this are g/mol.
• Working out moles

  
  

  $amount\ n=$$\frac{mass\ m}{molar\ mass\ M}$ $moles=$$\frac{mass\ }{Mr}$
• The molecular formula is the number of atoms of each element in a molecule.
• The empirical formula is the simplest whole-number ratio of atoms of each element in a compound.
• Relative molecular mass Mr compare the mass of a molecule with the mass of an atom of carbon-12
• Relative formula mass compares the mass of a formula unit with the mass of an atom of carbon-12.
• Working out empirical formulas from mass

  1. Convert mass into moles
  2. Find the smallest whole-number ratio by dividing by the smallest number of moles
  3. Write the empirical formula
• Working out molecular formulas from percentage mass

  1. Convert % mass into moles by dividing mass by Ar.
  2. Find the smallest whole-number ratio and empirical formula.
  3. Write the relative mass of the empirical formula.
  4. Find the number of empirical formulas in one molecule by dividing the relative molecular mass of the compound by the relative mass of the empirical formula.
  5. Find the molecular formula by multiplying the empirical formula by the number of empirical formulas in one molecule.
• Hydrated salts

  Many coloured crystals are hydrated- water molecules are part of their crystalline structure. This water is known as the water of crystallisation. When hydrated salts are heated to remove all the water, they are called anhydrous. The water of crystallisation is shown in the formula by a large dot between the compound formula and the water units.
• Working out the water of crystallisation

  1. Calculate the amount in moles of the anhydrous salt by dividing the mass by Mr
  2. Calculate the mass and amount in moles of water by dividing the mass by Mr
  3. Find the smallest whole-number ratio by dividing by the smallest number of moles
  4. Write down the formula of the hydrated salt
• How to work out the mass of an anhydrous salt

  1. Weigh an empty crucible
  2. Add the hydrated salt into the weighted crucible and weigh the crucible and the hydrated salt together
  3. Using a pipe clay triangle, support the crucible containing the hydrated salt on a tripod. Heat the contents of the crucible gently for about 3 minutes then strongly for 3 more minutes
  4. Leave the crucible to cool. Then weigh the crucible and anhydrous salt.