Required Practical 1

• describe how to make a standard solution

  1. weigh the weighing boat containing the solid on a 2dp balance

  2. transfer solid to beaker and reweigh weighing boat

  3. record the difference in mass

  4. add distilled water and stir with a glass rod until all the solid has dissolved

  5. transfer to a volumetric flask and add washings

  6. make up to the 250cm3 mark with distilled water

  7. shake flask
• why add washings?


  washings ensure any solution lost in transfer is kept to a minimum
• what are some possible errors with making standard solutions?

  ● If using anhydrous sodium hydrogensulfate (NaHSO4 ), make sure it is not too old as it will have picked up water and therefore mass values will not be as accurate.

  ● Balance - usually gives to 0.01g (systematic error - same for each value that is recorded).

  ● Alternatively the known mass of solid in the weighing bottle could be transferred to beaker, washed and washings added to the beaker.

  ● Remember to fill so the bottom of the meniscus sits on the line on the neck of the flask. With dark liquids like potassium manganate (K2 MnO4 ) it can be difficult to see the meniscus, so placing a piece of white paper behind to make the marker clearer to see.
• how do you reduce uncertainty when measuring mass?

  ● Using a more accurate balance or a larger mass will reduce the uncertainty in weighing a solid.

  ● Weighing sample before and after addition and then calculating difference (weighing by difference) will ensure a more accurate measurement of the mass added.
• 1. Pour approximately 100cm3 of the standard solution of known concentration into a beaker.

  2. Fill the burette with the standard solution of known concentration.

  3. Pour approximately 100cm3 of the solution with unknown concentration into a second beaker.

  4. Using a pipette filler and pipette to transfer exactly 25cm3 of solution into a 250cm3 conical flask.

  5. Add two to three drops of phenolphthalein indicator to the solution in the conical flask and note the initial colour of the indicator.

  6. Record the initial burette reading.

  7. Titrate the contents of the conical flask by adding solution to it from the burette until the indicator undergoes a definite, permanent colour change. Record the final burette reading in your table of results. Calculate the titre volume (change in volume in the burette).

  8. Repeat, calculate and record the volume of solution used in the titration in a table (titre volume). Repeat until two concordant results are obtained. Record all of the results that you obtain.
• how do you improve accuracy when filling the burette? explain why

  ● Rinse the burette before using a small volume of the solution.

  ● Make sure the jet space in the burette is filled and doesn't contain air bubbles.

  Explanation:

  If the jet space is not filled it will lead to errors if it then fills during the titration, leading to a larger than expected titre reading.
• how do you improve the accuracy of the conical flask and pipette?

  ● Rinse the 25cm3 pipette with the solution of unknown concentration.

  ● The conical flask should been rinsed with deionised water.
• why use a conical flask instead of a beaker?

  A conical flask is used in preference to a beaker because it is easier to swirl the mixture in a conical flask without spilling the contents.
• how do you improve the accuracy of the titre? explain why

  ● Add the solution slowly, swirling the flask gently to mix the solution.

  ● Add the solution dropwise near the end-point.

  explanation:

  Distilled water can be used during a titration to wash the sides of the flask so that all reactants are washed into the mixture. This water does not affect the titration as it doesn't change the number of moles of each reactant.
• what goes in the conical flask?

  the alkali
• how should you record and analyse results?

  ● lf 2 or 3 values are within 0.10cm3 and are therefore concordant, then the results are accurate and reproducible and the titration technique is good or consistent.

  ● Results should be clearly recorded in a table.

  ● Result should be recorded in full with both initial and final volume readings.

  ● Record titre volumes to 2dp (0.05 cm3).

  ● Only make an average titre volume using the concordant titre results.
• what are some safety precautions for this practical?

  ● Acids and alkalis are corrosive (at low concentrations acids are irritants).

  ● Wear eye protection and gloves.

  ● If spilled immediately wash affected parts after spillage.

  ● If a substance is unknown, treat it as potentially toxic and wear gloves.
• how do you reduce uncertainty in a titration?

  ● Replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the overall error.

  ● To reduce the uncertainty in a burette reading the titre volume needs to be made larger. This could be done by: - increasing the volume and concentration of the substance in the conical flask

  - decreasing the concentration of the substance in the burette.

  ● Leaving NaOH in the burette will cause damage to the apparatus which could lead to errors.
• What is a standard solution?

  a solution of known concentration
• What are concordance results?

  Titres that are within 0.1cm3 of each other