topic three chemistry

• $moles=$$mass/molar mass$
• molecular formulae


  the number and types of atoms of each element on a molecule
• empirical formula 


  the simplest whole number ratio of atoms of each element within a compound
• relative molecular mass (Mr)


  compares the mass of a molecule with the mass of a carbon 12 atom
• relative formula mas 


  compares the mass of a formula unit with the mass of a carbon 12 atom
• calculating molecular formula:
• calculating molecular formula :

  1. calculate the number of moles of each element
  2. calculate the empirical formula
  3. work out the relative formula mass ( Ar) using the empirical formula
  4. divide the Mr by the Ar
  5. multiply this value by the empirical formula to get the molecular formula
• number of moles = concentration x volume
• calculating the formula of a hydrated salt :

  1. calculate the moles of anhydrous salt formed
  2. calculate the moles of water of crystallisation
  3. find the ratio between these two moles
  4. this ratio casn be used to write the formula of the hydrated salt
• rtp is 20 degrees Celsius and 101kpa
• moles (at rtp)= volume of gas /24
• temperature in kelvin can be calculated by adding 273 to any temperature value in degrees celsius
• when the temperature and pressure is not rtp:

  pv=nrt pressure is in pa

  volume is in m cubed

  temperature is in kelvin
• cm to metres cubed is times by ten to the power of minus six
• dm cubed to metres cubed is times ten to the power of minus three
• kpa to pa is times ten to the power of three
• percentage yield = actual yield / theoretical yield x 100
• atom economy = sum of Mr of desired products/sum of Mr of all reactants x 100