C3- Quantitative Chemistry.

• Relative Formula Mass (Mr)


  Ar (relative atomic mass) x quantity

  • don’t use coefficient
• % mass of an element in a compound

  (Ar x number of atoms)/ Mr of compound x100
• Avogadro Constant


  6.02 x10^23
• Number of Moles=?


  Mass (g) / Mr of compund
• If mass is conserved, why may it change?

  Involvement of a gas
• Method to Balance equation using Moles:


  • work out Mr of compounds
  • Divide mass by Mr to calculate Moles
  • Divide each mole by smallest mole to find ratio
  • put ratio infront of compund
• Method of using moles to find limiting reactants:

  • write balanced equation
  • calculate Mr
  • calculate moles
  • use ratio to find other moles
  • use (mole= mass/mr rearranged ) to find mass
• Volume of gas (at rtp)

  (Mass of gas (g) / Mr of gas ) x 24
• One mole of any gas occupies 24dm^3 at 20 degrees celcius (at same temperature and pressure equal number of moles of any gas will occupy same volume)
• You can use the volume of one gas to find the volume of another using moles.
• Concentration (g/dm^3)


  mass(g) / volume (dm^3)
• Concentration (mol/dm^3)


  moles (mol)/ volume(dm^3)
• Converting cm^3 to dm^3

  /1000
• What’s atom economy?


  % of reactants forming useful products
• Atom economy (%)=


  (Mr of desired product/ Mr of all reactants) x100
• Disadvantages of Low Atom Economy:


  /

  • uses up resources quickly
  • makes a lot of waste materials that have to be disposed
  • unsustainable- raw materials will run out and waste has to go somewhere
  • aren’t usually profitable- raw materials often expensive to buy and can be expensive to responsibly remove and dispose waste products
• How to improve atom economy

  • find use for waste products
  • use reaction that gives useful by-products
• What does a 100% atom economy suggest?

  One product formed
• What does more products usually mean?

  Lower atom economy
• % yield 


  (mass of product made/ maximum theoretical mass of product) x100
• Why should industrial processes have higher percentage yields?

  Reduce waste and costs
• Why are yields always less than 100?

  product/ reactants always get lost

  • how depends on sort of reaction and apparatus used
• Common problems that decrease yield:

  
  

  • not all reactants make a product ; in reversible reactions products can turn back into reactants to yield is never 100
  • side reactions: reactants may react with air or impurities in the reaction mixture, so form extra products
  • products lost when separating: when you filter a solid from liquid, some solid will be left behind when scraped off and some liquid will be on solid as they stay wet
  • transferring material: material can be left behind in old container