c3- quantitative chemistry

• define Relative atomic mass

  the average mass of all the isotopes of that element
• what is the limiting reactant?

  The reactant that is completely used up in a chemical reaction and limits the amount of product that can be formed.
• describe the trend between the limiting reactant and the product formed


  the amount of product is directly proportional to the amount of limiting reactant

  e.g- if the limiting reactant is halved, then so is the product formed
• why is the amount of product formed directly proportional to the amount of limiting reactant?


  if you add more reactant there will be more reactant particles which leads to more product particles
• how would you calculate the amount of product formed by using the mass of the limiting reactant?


  1. write out a balanced equation
  2. work out the Mr of the reactant and product you NEED to know
  3. find out how many moles theres are of the substance you know the mass of
  4. use the balanced equation to work out how many moles there’ll be of the other substance
  5. use the number of moles to calculate the mass
• what does it mean for a reactant to be in excess?


  it does not all react, and has no affect on the amount of product formed
• what is a mole?


  unit we use to measure the amount of chemical we have
• what is avogadro constant?


  6.02 x 10^23
• why is avogadros constant used?

  to show that the mass of 6.02x10^23 of any substance will be equal to the substances Ar

  e.g- 1 mole of carbon=12g
• what is %yield?


  the amount of product you actually make as a % of the amount you should theoretically make
• why dont chemical reactions have a high yield of 100%?

  1. the raw materials (reactants) may not be pure
  2. some of the product may be left behind in the apparatus
  3. the reaction may not have fully completed
  4. some reactants may give some unexpected products
• what is atom economy?


  the mass of the product you want as a % of the mass of all the products you make
• how do you work out the atom economy?


  relative formula mass of desired products / relative formula mass of all reactants x 100
• how can you maximise % yield?

  use the most efficient conditions + apparatus to reduce energy use, cost and conserve raw materials (as they are scarce, limited and expensive)
• how do you maximise atom economy?


  choosing the most efficient reaction to make the product in order to reduce waste and pollution
• how do you calculate the % yield?


  mass of product made / maximum theoretical mass of product x 100
• what does conservation of mass state?


  no atoms are lost nor made during a chemical reaction

  (so mass stays the same)
• what is a chemical change?


  A rearrangement of the atoms in the reactants to form the products
• how do you work out the volume of a gas?

  mass of gas / Mr of gas x 24

  OR

  moles x 24
• why do we times the volume of a gas by 24?


  one mole of any gas occupies 24dm^3 at 20*C (room temperature)
• at the same temperature and pressure, equal numbers of moles of any gas will occupy the same volume
• how do you work out concentration using units of mass?


  Mass of solute / volume of solvent
• how do you find out the concentration using moles?


  number of moles / volume of solvent