Amount of Substance

• Weight of atoms

  • mass is relative to C-12 -> 1/12 of C-12 = exactly 1
  • relative atomic mass (Ar) = average mass of an atom relative to 1/12 of the mass of a carbon-12 atom
  • relative molecular mass (Mr) = mass of a molecule compared to 1/12 the relative atomic mass of a carbon-12 atom
  • 1 mole = 6.022 x 10^23 particles of a substance
  • mass of 1 mole of a substance = Ar/Mr in grams
• formula: Mass of 1 atom/ion


  mass = Mr / 6.022 x 10^23
• formula: moles

  moles (mol) = Mass (g) / Mr
• formula: concentration

  concentration (moldm-3) = moles (mol) / volume (dm3)
• Maths question: finding the reacting moles

  1. write out formula
  2. write in table
  3. Use molar ratio
• Maths question: finding the limiting reactant

  limiting reactant = the reactant that runs out first stopping the reaction go any further

  1. write out formula
  2. write in table
  3. use molar ratio
• Maths question: Back titration

  2 reactions -> 1st = titration, 2nd = titration with leftover acid from first reaction

  1. write equations
  2. find moles of substance only in second equation
  3. find moles of shared substance
  4. calibrate it
  5. find moles of the shared substance at the start
  6. answer question
• formula: percentage mass

  (mass of element) x (no. of moles of element) / mass of molecule * 100
• Maths question: empirical formula

  empirical formula = simplest whole number ratio of the atom in a compound

  1. write out all elements in compound
  2. find Mr
  3. calculate moles
  4. divide all moles by the smallest value of moles

  if it's in percentages - out of 100g
• Maths question: water of crystallisation

  water of crystallisation = water molecules in a compound

  1. write out the rest of the compound & the water separately
  2. write out mass & Mr
  3. calculate moles
  4. divided moles by smallest value of moles
• formula: Ideal Gas Equation

  pV = nRT

  p = pressure (Pa)

  V = volume (m^3)

  n = no. of moles (mol)

  R = gas constant (8.314 J/K/mol)

  T = temperature (K)

  1000cm3 = 1dm3 -> 1000dm3 = 1m3
• formula: percentage yield

  percentage yield = measure of actual yield compared to maximum theoretical yield

  % yield = (actual yield / theoretical yield) x 100

  reactions are rarely 100%: side reactions, incomplete reactions, loss of product in transfer
• formula: atom economy

  atom economy = measure of 'amount' of useful, desired product that is made from the reactants in a reaction

  atom economy = (Mr of desired product / Mr of total reactants) x 100
• formula: uncertainty

  uncertainty = uncertainty of the measurement compared to the value of the measurement

  uncertainty = (error / measured) x 100

  reduce % error/uncertainty by increasing the value measured
• Required practical 1: Making a standard solution pt 1

  titration = reacting a solution of known concentration with solution of unknown concentration to find the concentration

  Method: making a standard solution

  1. Calculate mass of required substance needed to make 250 cm3 of a 0.100 mol dm-3 solution
  2. Weigh clean weighing boat on a 2dp balance
  3. Place weighing boat on balance & add mass of required substance using a spatula
  4. Pour contents of weighing boat into beaker & re-weigh the weighing boat & record it's value - calculate the mass of substance transferred
• Required practical 1: Making standard solution pt 2

  Method: making a standard solution

  1. Add approximately 100cm3 of deionised/distilled water to beaker containing solid & use glass stirring rod to dissolve solid
  2. use funnel to add contents of beaker to volumetric flask & use water to rinse funnel, rod & beaker
  3. make up volumetric flask to graduated mark by carefully adding the water
  4. Stopper the volumetric flask & invert to combine fully
• Required practical 1: Titration pt 1

  Method: titration

  1. Pour 100cm3 of standard solution with known conc. to beaker
  2. Fill burette with standard solution with known conc.
  3. Pour 100cm3 of solution with unknown conc. into a second beaker
  4. Use pipette filler & pipette to transfer exactly 25cm3 of solution into a 250cm3 conical flask & add indicator
• Required practical 1: titration pt 2

  Method: titration

  1. record initial burette reading
  2. Titrate contents of conical flask until indicator undergoes definite, permanent colour change. Record final burette reading & calculate final titre volume
  3. Repeat until 2 concordant (within 0.1 of each other) results are obtained
• Errors in titrations

  • leaving filter funnel on top -> residue can drip -> start volume increases
  • bubble under tap -> decreases start volume
  • not using a conical flask -> may lead to loss of liquid
  • not swirling constantly -> chemicals not fully mixed -> increases final titre
  • not washing sides of conical flask just before end point -> solution of sides due to splashback