topic 3-quantitative chemistry

• Relative atomic mass


  The mass of an atom relative to the mass of a carbon-12 atom
• There are two chlorine atoms in MgCl2, so the relative atomic mass of chlorine needs to be multiplied by 2
• Percentage mass of an element in a compound

  Calculated using the formula: (Ar x number of atoms of that element) / Mr of the compound x 100
• Mole


  The amount of a substance that contains 6.02 x 10^23 particles (atoms, molecules, or ions)
• The mass of one mole of any substance is equal to its relative atomic or formula mass in grams
• In a balanced chemical equation, the total mass of reactants equals the total mass of products
• Limiting reactant


  The reactant that is completely used up in a reaction, limiting the amount of product formed
• The amount of product formed is directly proportional to the amount of limiting reactant
• Mole of any gas

  Occupies 24 dm³ at 20°C and 1 atm pressure
• Limiting reactant

  The reactant that determines the amount of product formed in a reaction
• Calculating mass of product formed


  Use mass of limiting reactant and balanced reaction equation
• Mole of any gas

  Occupies 24 dm³ at 20 °C and 1 atm
• Calculating volume of gas


  Mass of gas / Molar mass of gas * 24
• Calculating volume of gas in reactions


  Use volume of one gas to find volume of another using mole ratios
• Concentration


  Measure of how crowded things are in a solution
• Calculating concentration in g/dm³

  Mass of solute / Volume of solvent
• Calculating concentration in mol/dm³


  Moles of solute / Volume of solvent in dm³
• Using concentrations to find unknown concentration


  Moles of unknown * Concentration of known = Moles of known * Concentration of unknown
• Atom economy


  Percentage of reactant atoms that end up in desired product
• Reaction to produce hydrogen

  • CH4(g) + H2O(g) → CO(g) + 3H2(g)
• High atom economy is better for profits and the environment
• Percentage yield


  Actual mass of product made / Theoretical maximum mass of product * 100
• Yields are always less than 100% due to side reactions, incomplete reactions, and losses during separation