Quantitative Chemistry

• Whenever you are measuring something there is going to be a degree of uncertainty whether it's a burette, a measuring cylinder or a beaker
• You need to look for the bottom of the meniscus when measuring as there is a difference between where it looks at the top and where it is at the bottom
• You can never say the measurement is accurate because it might be a quarter of the way or three quarters of the way to the next line
• Concentration


  Amount divided by volume, measured in moles per decimeter cubed
• Balancing an equation


  1. List reactants and products

  2. Circle compounds

  3. Count numbers of each element

  4. Adjust coefficients to balance
• Relative formula mass (Mr)


  The sum of the relative atomic masses of the atoms in a molecule
• Mole


  The unit for the amount of a substance, equal to 6.02 x 10^23 atoms, ions or molecules
• Calculating moles


  Moles = Mass / Mr
• Balancing an equation for hydrogen peroxide decomposition

  1. Write initial unbalanced equation

  2. Balance by adjusting coefficients

  3. Final balanced equation: 2H2O2 → 2H2O + O2
• Limiting reactant


  Reaction continues using up the limiting reactant to form products until it is used up
• Percentage yield


  Actual yield / Theoretical yield x 100
• Titration procedure


  1. Use pipette to add known volume of solution

  2. Add indicator

  3. Slowly add acid from burette until first permanent colour change

  4. Record final burette reading

  5. Repeat until consistent titres
• Calculating titration results


  1. Find moles of acid from volume and concentration

  2. Moles of acid = moles of alkali due to neutralisation

  3. Calculate concentration of alkali
• Molar volume of gases

  1 mole of any gas occupies 24 dm^3 at room temperature and pressure
• There is a lot of maths in quantitative chemistry, including balancing equations, calculating concentrations, moles, percentage yields, and gas volumes
• The book "Maths for Chemistry Bits" has 60 equations to practice balancing, plus lots of other quantitative chemistry calculations