Topic 3 - quantative chem

• Relative formuala mass is the sum of all the relative atomic masses of the elements in the compound
• equation for percentage mass of an element in a compound 


  atomic mass (of that element) x number of atoms of that element / relative formula mass of the compound
• what is avagadros constant ?


  6.02 x 10^23
• equation for number of moles 


  mass in g (of an element or compound) / Mr (of the element or compound)
• What is the law of conservation of mass


  in a chemical reaction the total mass of the reactants in a chemical reaction is equal to the total mass of the products (mass is neither created or destroyed )
• Limiting reactanst

  • a reactions stops when all of one of the reactants is used up , any other reactant is in excess.
  • excess reactants are usually added to make sure the other reactant is used up
  • the reactant that is used up in a reaction is called the limiting reactant
  • the amount of product formed is directly proportional to the amount of limiting reactant
• Calculating the mass of the product formed (using mass of the limiting reactant) 


  • write out the balanced equation
  • work out relative formula mass (Mr) of the reactant and the product you want
  • find out how many moles of the substance you know the mass of
  • use balanced equation to find how many moles of the other subatance there will be
  • use number of moles to calculate mass
• one mole of any gas occupies 24$dm^3$ at room temp (20C)
• Volume of gas = (mass of gas / Mr of gas) x 24
• the amount of substance (e.g. mass or number of moles) in a certain volume of solution is called its concentration
• the more solute (substance that is disolved ) there is in a given volume the more concentrated the solution
• concentration = mass of solute (g) / volume of solvent ($dm^3$)
• concentration = number of moles of solute (mol) / volume of solvent ($dm^3$)
• what is atom economy 


  percentage of reactants forming useful products
• atom economy = 100 x (mass of desired product / mass of all reactants)
• why is low atom economy bad (for the environment)


  • use up resources very quiclly
  • make lots of waste materials that need to be disposed of
  • this makes recations unsistainable - the raw materials will run out and the waste has to go somewhere
  • for same reasons low atom economy reactions are not as profitable
• the reactions with the highest atom economy are the ones that only have one product (they have an atom economy of 100%), the more products there are te lower the atom economy is likely to be
• the amount of product you get is known as the yield
• percentage yield = (mass of product actually made / maximum theoretical mass of the product) x 100
• reasons why you never get a 100% yield 


  • not all reactants react to make a product (e.g. in reversible reactions the products can turn ack into reactants so the reactants will never all be used up )
  • there might be side reactions- may end up forming extra products other than the ones you want
  • you lose some product when you seperate it from the reaction mixture - product may be left on containers or apparatus