Practicals

• Removing water of crystallisation in calcium sulfate crystals

  1. Weigh 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
• Small amounts of the solid, such as 0.100 g, should not be used in this experiment as the percentage uncertainties in weighing will be too high
• Large amounts of hydrated calcium sulfate, such as 50g, should not be used in this experiment as the decomposition is likely to be incomplete
• Lid


  Improves the accuracy of the experiment as it prevents loss of solid from the crucible but should be loose fitting to allow gas to escape
• The crucible needs to be dry otherwise a wet crucible would give an inaccurate result. It would cause mass loss to be too large as the water would be lost when heating
• Heating in a crucible


  This method could be used for measuring mass loss in various thermal decomposition reactions and also for mass gain when reacting magnesium in oxygen
• Hazardous substances


  • Irritant - dilute acid and alkalis- wear googles
  • Corrosive- stronger acids 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 / easily oxidised materials
• Hazardous substances in low concentrations or amounts will not pose the same risks as the pure substance
• Potential errors in using a gas syringe

  • Gas escapes before bung inserted
  • Syringe sticks
  • Some gases like carbon dioxide or sulphur dioxide are soluble in water so the true amount of gas is not measured
• The volume of a gas depends on pressure and temperature so when recording volume it is important to note down the temperature and pressure of the room
• Using a gas syringe to calculate the Mr of propanone

  1. Extract 0.20 cm3 of propanone into a hypodermic syringe and then measure the mass of this syringe

  2. Using hand protection, remove a gas syringe from the oven and note the volume of air already in the barrel – about 5 cm3

  3. Inject the propanone through the self-seal cap into the barrel

  4. Put the gas syringe back into the oven

  5. Measure the mass of the empty hypodermic syringe immediately

  6. After a few minutes measure the volume of the gas in the gas syringe, record the temperature of the oven shelf and the pressure of the room
• Moles of gas can be calculated from gas volume (and temperature and pressure) using ideal gas equation PV = nRT
• Making a solution


  1. Weigh the sample bottle containing the required mass of solid on a 2 dp balance

  2. Transfer to beaker

  3. Reweigh empty sample bottle

  4. Record the difference in mass

  5. Add 100cm3 of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid

  6. Pour solution into a 250cm3 graduated flask via a funnel

  7. Rinse beaker and funnel and add washings from the beaker and glass rod to the volumetric flask

  8. Make up to the mark with distilled water using a dropping pipette for last few drops

  9. Invert flask several times to ensure uniform solution
• Graduated/volumetric flask


  A graduated flask has one mark on the neck which the level to fill to get the accurate volume. Do not heat or put hot solutions in the volumetric flask because the heat would cause the flask to expand and the volume would then be incorrect
• Diluting a solution


  1. Pipette 25.0cm3 of original solution into a 250cm3 volumetric flask

  2. Make up to the mark with distilled water using a dropping pipette for last few drops

  3. Invert flask several times to ensure uniform solution
• Using a volumetric pipette is more accurate than a measuring cylinder because it has a smaller uncertainty
• Measuring mass accurately


  1. Measure mass on 2 or 3d.p. balance of a weighing bottle with the required quantity of solid in it

  2. Empty mass into reaction vessel/flask

  3. Reweigh the now empty weighing bottle

  4. Subtract the mass of the empty weighing bottle from the first reading to give exact of mass actually added
• Titration


  Done often to find out the concentration of one substance by reacting it with another substance of known concentration
• Detailed Method for Titration

  1. Rinse pipette with substance to go in it (often alkali)

  2. Pipette 25 cm3 of solution A into conical flask

  3. Touch surface of solution with pipette (to ensure correct amount is added)

  4. Rinse equipment (burette with acid, pipette with alkali, conical flask with distilled water)

  5. Add a few drops of indicator and refer to colour change at end point

  6. Add solution from burette whilst swirling the mixture and add drop-wise at end point

  7. Note burette reading before and after addition of solution

  8. Repeats titration until at least 2 concordant results are obtained- two readings within 0.1 of each other
• Using the burette


  • The burette should be rinsed out with substance that will be put in it
  • Make sure the jet space in the burette is filled with the solution and air bubbles are removed
  • Read the bottom of the meniscus on the burette
• Adding indicator


  • Phenolphthalein: pink (alkali) to colourless (acid), end point pink colour just disappears
  • Methyl orange: yellow (alkali) to red (acid), end point orange
• Use a white tile underneath the flask to help observe the colour change
• Indicators are generally weak acids so only add a few drops of them. If too much is added they will affect the titration result
• Recording results


  • Results should be clearly recorded in a table
  • Result should be recorded in full (i.e. both initial and final readings)
  • Record titre volumes to 2dp (0.05 cm3)
• Working out average titre results

  • Only make an average of the concordant titre results
  • If 2 or 3 values are within 0.10cm3 and therefore concordant or close then we can say results are accurate and repeatable and the titration technique is good and consistent
• Titration procedure


  1. Repeat titration until at least 2 concordant results are obtained (two readings within 0.1 of each other)

  2. Add distilled water to conical flask during titration to wash sides of flask so all acid is washed into reaction mixture

  3. Distilled water does not affect titration reading as it does not react with reagents or change moles of acid added

  4. Only use distilled water to wash conical flasks between titrations as it does not add extra moles of reagents
• Recording results


  • Results should be clearly recorded in a table
  • Record titre volumes to 2dp (0.05 cm3)
  • Only make an average of the concordant titre results (2 or 3 values within 0.10cm3)
• A single titration could be flawed. Repeating allows for anomalous titres to be spotted and discounted
• In quality control it will be necessary to do titrations/testing on several samples as the amount/concentration of the chemical being tested may vary between samples
• If titrating a mixture to work out the concentration of an active ingredient it is necessary to consider if the mixture contains other substances that have acid base properties
• Common Titration Equations


  • CH3CO2H + NaOH CH3CO2-Na+ + H2O
  • H2SO4 + 2NaOH Na2SO4 +2H2O
  • HCl + NaOH NaCl+H2O
  • NaHCO3 + HCl NaCl + CO2 + H2O
  • Na2CO3 + 2HCl 2NaCl + CO2 + H2O
• Detailed Procedure: how much iron is in iron tablets

  1. Weigh accurately two 'ferrous sulphate' tablets

  2. Grind up the tablets with a little 1 mol dm-3 sulphuric acid, using a pestle and mortar

  3. Transfer the resulting paste into a 100cm3 volumetric flask using 1 M sulphuric acid to rinse

  4. Make up the solution to exactly 100cm3 with 1 mol dm-3 sulphuric acid

  5. Titrate 10.0 cm3 portions of the solution with 0.0050 M potassium manganate(VII)
• Manganate Redox Titrations


  • The redox titration between Fe2+ with MnO4- (purple) is a very common exercise
  • The purple colour of manganate can make it difficult to see the bottom of the meniscus in the burette
  • If the manganate is in the burette then the end point of the titration will be the first permanent pink colour
• Choosing correct acid for manganate titrations

  • Only use dilute sulfuric acid as it can supply the 8H+ ions needed
  • Insufficient volumes of sulfuric acid will mean the solution is not acidic enough and MnO2 will be produced instead of Mn2+
  • Weak acids like ethanoic acid cannot supply the large amount of hydrogen ions needed
  • Conc HCl cannot be used as the Cl- ions would be oxidised to Cl2 by MnO4-
  • Nitric acid cannot be used as it is an oxidising agent and oxidises Fe2+ to Fe3+
• The reaction between MnO4- and C2O42- is slow to begin with (as the reaction is between two negative ions). To do as a titration the conical flask can be heated to 60o C to speed up the initial reaction
• Calculating Apparatus Uncertainties


  • Each type of apparatus has a sensitivity uncertainty (e.g. balance 0.001 g, volumetric flask 0.1 cm3, burette 0.15 cm3)
  • Calculate the percentage error for each piece of equipment used
  • To calculate the maximum total percentage apparatus uncertainty in the final result add all the individual equipment uncertainties together
• Reducing uncertainties in a titration

  • Replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the % uncertainty
  • To reduce the % uncertainty in a burette reading it is necessary to make the titre a larger volume
• If the %uncertainty due to the apparatus < percentage difference between the actual value and the calculated value then there is a discrepancy in the result due to other errors
• If the %uncertainty due to the apparatus > percentage difference between the actual value and the calculated value then there is no discrepancy and all errors in the results can be explained by the sensitivity of the equipment
• Readings and Measurements


  • Readings are the values found from a single judgement when using a piece of equipment
  • Measurements are the values taken as the difference between the judgements of two values (e.g. using a burette in a titration)