C3: 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 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. Check numbers of each element

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


  Sum of the relative atomic masses of the atoms in a molecule
• Mole


  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 unbalanced equation

  2. Balance by adjusting coefficients

  3. Hydrogen peroxide decomposes to water and oxygen
• Limiting reactant


  Reaction continues until limiting reactant 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
• Titration calculations


  Use moles of acid = moles of alkali to calculate concentration
• Gases


  1 mole occupies 24 dm^3
• There is a lot of maths in quantitative chemistry, including balancing equations, titration calculations, bond enthalpy calculations etc.