Chemistry-C3 quantitative chemistry

• Law of conservation of mass


  Total mass of reactants = total mass of products
• When magnesium reacts with oxygen to form magnesium oxide


  The mass increases because oxygen atoms have been joined to the magnesium that hadn't been weighed beforehand
• When calcium carbonate reacts with acid and makes carbon dioxide


  The mass decreases because carbon dioxide is a gas and escapes from the reaction
• Relative formula mass, Mr


  The sum of the masses of each atom in a compound
• State symbols


  • (s) solid
  • (l) liquid
  • (g) gas
  • (aq) aqueous - dissolved in water
• Excess


  We have some of this substance left over, unreacted, after the reaction (we had more than we needed)
• Limiting reactant


  A substance that is completely used up, or reacted, in a reaction
• Formula relating concentration, mass and volume


  Concentration = mass (in g) / volume (in dm3)
• Formula to calculate mass


  Mass (in g) = concentration x volume (in dm3)
• Conversion between cm3 and dm3

  1 dm3 = 1000 cm3
• Converting 25 cm3 to dm3

  25 cm3 = 0.025 dm3 (divide by 1000)
• Avogadro's number


  6.02 x 10^23
• Formula relating moles, mass and Mr

  Moles = mass / Mr
• Formula to calculate mass

  Mass = moles x Mr
• There are 3 different elements present in the formula H2SO4
• There are 7 different atoms present in the formula H2SO4
• Balancing chemical equations


  2 Ca + O2 → 2 CaO

  2 Al + 3 Br2 → 2 AlBr3

  4 K + O2 → 2 K2O

  C5H12 + 8 O2 → 5 CO2 + 6 H2O
• Calculating formula mass, Mr


  CO2 = 44

  H2SO4 = 98

  Mg(NO3)2 = 148
• Calculating mass of solid dissolved in 25 cm3 of 12 g/dm3 solution

  Mass = concentration x volume = 12 x 0.025 = 0.3 g
• The melting points of Group 1 metals decrease as the atomic number increases
• Substances and their descriptions


  • Lithium oxide - compound

  Oxygen - element

  Lithium - metal

  Lithium oxide - mixture
• Balancing the equation for the reaction of lithium with oxygen

  4 Li + O2 → 2 Li2O
• Calculating the relative formula mass (Mr) of lithium oxide (Li2O)

  Mr = (2 x 7) + (1 x 16) = 30
• Calculating the mass of lead nitrate dissolved in 25 cm3 of 6 g/dm3 solution

  Mass = concentration x volume = 6 x 0.025 = 0.15 g
• Atoms have a positively charged nucleus
• Mass is concentrated in the nucleus in the centre of atoms
• Calculating the diameter of one gold atom

  The gold foil is 4.00 x 10^-7 m thick and 2400 atoms thick, so the diameter of one gold atom is:

  4.00 x 10^-7 m / 2400 = 1.67 x 10^-10 m (3 s.f.)
• Alpha particle scattering experiment


  • Alpha particles are fired at gold foil
• Conclusions from alpha particle scattering experiment


  • Atoms have a positively charged nucleus
  • Mass is concentrated in the nucleus in the centre of atoms
• Gold foil


  • 4.00 × 10–7 metres thick
  • 2400 atoms thick
• Calculating diameter of one gold atom


  1. Divide thickness by number of atoms

  2. Convert to 3 significant figures
• Diameter of one gold atom (3 significant figures)
• Gold reacts with elements in Group 7

  Equation: 2 Au + 3 Cl2 → 2 AuCl3
• Calculating mass of chlorine needed to react with 0.175 g of gold


  1. Convert moles of gold to moles of chlorine

  2. Calculate mass of chlorine using molar mass
• Mass of chlorine
• Calcium reacts with fluorine


  Produces calcium fluoride (CaF2)
• Oxidation and reduction


  • Calcium loses electrons (oxidation)
  • Fluorine gains electrons (reduction)
• Calcium fluoride

  • Has a high melting point
• Reason for high melting point of calcium fluoride
• Calculating mass of sulfur hexafluoride produced


  1. Convert moles of fluorine to moles of sulfur hexafluoride

  2. Calculate mass of sulfur hexafluoride using molar mass