amount of substance

• mole is amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12
• relative atomic mass is average mass of an atom relative to 1/12th the mass of an atom of carbon-12
• relative molecular mass is the average mass of a molecule relative to 1/12th the mass of an atom if carbon-12
• mole calculation for pure solids, liquids and gases

  moles (mol) = mass (g) /Mr
• mole calculation for gases

  PV=nRT

  pressure in Pa

  volume in m3

  r is 8.31

  temperature in K
• to convert temperature from degrees Celsius to kelvin add 273
• mole equation for solutions

  concentration (mol dm-3) = moles (mol) / volume (dm3)
• converting volumes

  cm3 to dm3 = divide by 1000

  cm3 to m3 = divide by 1 000 000

  dm3 to m3 = divide by 1000
• 1000mg = 1g

  1000g = 1kg

  1000kg = 1 tonne
• number of particles = moles of substance (mol) x Avogadro's constant (6.022x10^23)
• density (g cm-3) = mass (g) / volume (cm3)
• empirical formula is the simplest ratio of atoms of each element in a compound
• working out empirical formula

  1. divide each mass by atomic mass of element
  2. use answers from step 1 and divide by smallest numbers
  3. multiply to give whole numbers if needed
• molecular formula is the actual number of atoms of each element in a compound
• work out molecular formula from the relative molecular mass (Mr) and work out how many times the mass of the empirical formula fits into the Mr
• hydrated salt contains water of crystallisation
• heating in a crucible can be used to measure mass loss in various thermal decomposition reactions and also for mass gain when reacting magnesium in oxygen
• heating in a crucible steps

  1. weigh an empty clean dry crucible and lid
  2. add 2g of hydrated calcium sulfate to crucible and reweigh
  3. heat strongly with a bunsen for a few minutes
  4. weigh crucible and contents again
  5. continue heating and weighing until a constant mass is reached as the reaction is complete
• heating in a crucible notes

  lid improves accuracy as it prevents solid loss from the crucible but should be loose fitting so gas can escape

  don't use large amount of salt as decomposition incomplete but not too small as high percentage uncertainty in weighing

  if crucible was wet not dry, mass lost would be higher as water is lost when heating
• solution is a mixture formed when a solute dissolves in a solvent
• mass concentration (g dm-3) = mass (g) / volume (dm3)
• conc in g dm-3 = conc in mol dm-3 x Mr
• when soluble ionic solids dissolve in water they will dissociate into separate ions

  this can lead to the concentration of ions differing from the concentration of the solute
• weighing by difference

  1. weigh sample bottle with required mass of solid on 2dp balance
  2. transfer to beaker and reweigh bottle
  3. record difference in mass
• making a solution

  1. weigh by difference and add to beaker
  2. add 100cm3 of distilled water and use glass stirring rod to dissolve
  3. pour solution into 250cm3 graduated flask via funnel
  4. rinse beaker and funnels, add washings to volumetric flask
  5. make up to mark with distilled water and use pipette near end point
  6. invert flask to ensure uniform solution
• making a solution notes

  is solid does not dissolve, heat gently

  can add weighed solid and add washings instead

  fill so the bottom of the meniscus sits on the line on the neck of the flask
• diluting a solution steps

  1. pipette 25cm3 of original solution into 250cm3 volumetric flask
  2. make up to mark with distilled water and use pipette for the last few drops
  3. invert flask several times for uniform solution
• dilution notes

  use a volumetric pipette and not measuring cylinder as it has smaller uncertainty

  use teat pipette to make up to mark in volumetric flask to ensure volume of solution is accurately measured and does not go above the line
• new diluted concentration = original concentration x (original volume / new diluted volume)
• irritant- dilute acid and alkali

  wear goggles
• corrosive- stronger acid and alkalis

  wear goggles
• flammable- keep away from naked flames
• toxic- wear gloves, avoid skin contact, wash hands after use
• oxidising- keep away from flammable or easily oxidised materials
• volume of gas depends on pressure and temperature
• potential errors in using a gas syringe:

  gas escapes before bung inserted

  syringe sticks

  some gases like carbon dioxide or sulfur dioxide are soluble in water so the true amount of gas is not measured
• when drawing gas syringe don't leave gaps where gas can escape and draw measurement markings on barrel

  
• percentage yield = actual yield / theoretical yield x 100
• percentage atom economy=

  mass of useful products / mass of all reactants x 100
• chemists want high percentage yield as it means efficient conversion of reactants to products