amount of substance

• Mole


  The amount of substance in grams that has the same number of particles as there are atoms in 12 grams of carbon-12
• Avogadro's Number


  There are 6.022 x 10^23 atoms in 12 grams of carbon-12
• One mole of any specified entity contains 6.022 x 10^23 of that entity
• Equations used for most calculations at A-level

  • For pure solids, liquids and gases
  • For gases: PV = nRT
  • For solutions: concentration = moles/volume
• Relative atomic mass


  The average mass of one atom compared to one twelfth of the mass of one atom of carbon-12
• Relative molecular mass


  The average mass of a molecule compared to one twelfth of the mass of one atom of carbon-12
• Using Equation 1


  1. Calculate the number of moles of CuSO4 in 35.0g of CuSO4

  2. Calculate the number of moles in 75.0mg of CaSO4.2H2O
• Significant Figures


  Give your answers to the same number of significant figures as the number of significant figures for the data given in a question
• Avogadro's Constant


  1. Calculate the number of atoms of tin in a 6.00 g sample of tin metal

  2. Calculate the number of chloride ions in a 25.0 cm3 of a solution of magnesium chloride of concentration 0.400 mol dm-3

  3. Calculate the number of molecules of ethanol in a 0.500 dm3 of ethanol liquid
• Density


  Calculate the density of gold in a bar measuring 10 cm by 20 cm by 50 cm with 980 mol of pure gold
• Empirical formula


  The simplest ratio of atoms of each element in the compound
• Molecular formula


  The actual number of atoms of each element in the compound
• Heating in a crucible

  1. Weigh an empty clean dry crucible and lid

  2. Add 2g of hydrated calcium sulfate to the crucible and weigh again

  3. Heat strongly with a Bunsen for a couple of minutes

  4. Allow to cool

  5. Weigh the crucible and contents again

  6. Heat crucible again and reweigh until you reach a constant mass
• Hydrated salt


  Contains water of crystallisation
• Calculating concentration of solutions

  Calculate the concentration of solution made by dissolving 5.00 g of Na2CO3 in 250 cm3 water
• A solution is a mixture formed when a solute dissolves in a solvent
• Molar concentration can be measured for solutions
• Ration


  mol dm-3 or M
• Converting volumes


  1. cm3 to dm3 ÷ 1000

  2. cm3 to m3 ÷ 1000 000

  3. dm3 to m3 ÷ 1000
• Example 11


  • Calculating concentration of solution made by dissolving 5.00 g of Na2CO3 in 250 cm3 water
• cm3


  Equivalent to a cube 1cm x 1cm x 1cm
• dm3


  Equivalent to a cube 10cm x 10cm x 10cm = 1000 cm3
• m3


  Equivalent to a cube 100cm x 100cm x 100cm = 1000000 cm3
• 1 cm3 = 1 mL
• 1 dm3 = 1 litre
• To convert cm3 into dm3 divide by 1000
• To convert m3 into dm3 multiply by 1000
• Solution


  Mixture formed when a solute dissolves in a solvent
• Molar concentration


  Calculated by dividing the amount in moles of the solute by the volume of the solution, measured in dm3. Unit is mol dm-3 or M
• Example 12


  • Calculating concentration of solution made by dissolving 10 kg of Na2CO3 in 0.50 m3 water
• Concentration in g dm-3

  Calculated by multiplying concentration in mol dm-3 by Mr of the substance
• Mass concentration


  Concentration of a solution measured in terms of mass of solute per volume of solution. Unit is g dm-3
• Ions dissociating


  1. Soluble ionic solids dissolve in water and dissociate into separate ions

  2. Concentration of ions can differ from concentration of solute
• Example 13


  • Dissolving 5.86g (0.1 mol) of sodium chloride (NaCl) in 1 dm3 of water
• Example 14


  • Dissolving 9.53g (0.1 mol) of magnesium chloride (MgCl2) in 1 dm3 of water
• Making a solution


  1. Weigh sample bottle

  2. Transfer to beaker and reweigh

  3. Record difference in mass

  4. Add 100 cm3 distilled water and stir

  5. Pour solution into 250 cm3 volumetric flask via funnel

  6. Rinse beaker and funnel, add washings to flask

  7. Make up to mark with distilled water
• Remember to fill so the bottom of the meniscus sits on the line on the neck of the flask
• Use a teat pipette to make up to the mark in volumetric flask to ensure volume of solution accurately measured
• Diluting a solution

  1. Pipette 25 cm3 of original solution into 250 cm3 volumetric flask

  2. Make up to the mark with distilled water
• Using a volumetric pipette is more accurate than a measuring cylinder because it has a smaller uncertainty