mass, moles and Mr

• what is Avogadro's constant?

  1 mole of any substance is 6.022 x 10^23 atoms
• what is the calculation for the mass of a species?

  mass of a species = mass of 1 mole / Avogadro's constant
• what is the calculation for the number of a species?

  number of species = number of moles x Avogadro's constant
• what is the calculation for mass?

  mass = moles x Mr
• example: what mass of oxygen is present in 140g of CaCO3?

  moles of CaCO3 - 140 / 100.1 = 1.3986 moles

  moles of O3 - 1.3986 moles

  mass of O3 - 1.3986 x 48 = 67.13g
• what does in excess mean?


  one reactant is in excess, the other reactant will react completely and be used up
• what does reacts completely mean? 


  all of this reactant will be used up in a reaction
• what does limiting reagent mean?


  opposite of 'in excess', all the limiting reactant will be used up in a reaction and the reaction stops
• what does stoichiometry mean?


  molar ratio
• example: if 10g of Pb(NO3)2 is mixed with 12g of KI, which reactant is in excess?

  Pb(NO3)2 + 2KI --> 2KNO3 + PbI2

  moles of Pb(NO3)2 - 10 / 331.2 = 0.0302 moles

  moles of KI - 12 / 166 = 0.0723 moles

  0.0302 x 2 = 0.0604

  0.0723>0.0604

  KI is in excess