5. This 0.02 mol dm-3 of hydrochloric acid concentration was made up by taking 2.5cm3 of the 2.00 mol dm-3 hydrochloric acid using a pipette and was then transferred into a volumetric flask where I made the total volume up to 250cm3 with water. Using a pipette to measure the acid ensures accurate results, as a pipette ensures a high degree of precision. Transferring the acid then into a volumetric flask also contributes to the reliability of my results as a volumetric flask has a mark that measures to 250cm3, this allowed me to accurately fill the flask up to the 250cm3 with water. Therefore, this ensures that you have the precise volume of solution. (See method on how to use a pipette accurately- see appendix 1)
6.To measure the concentration of the limewater we used titration with hydrochloric acid. Titration is the measurement of how much of one reagent of known concentration is needed to react with another. Therefore, we will be measuring how much hydrochloric acid of known concentration (2.00 mol dm-3) is needed to react with the calcium hydroxide. This is when the solution becomes neutral i.e. pH7. (See method on how to titrate accurately – see appendix 2)
7. So after finding a suitable dilution of hydrochloric acid to use which was 0.02 mol dm3, the burette was filled with 50cm3 of hydrochloric acid, using a measuring cylinder that measures to 1cm3. We had to make sure that there were no air bubbles present in the burette jet, this would have affected the volume of liquid in this method. To avoid this we kept the burette jet running until we could see clear that there were no air bubbles present in the burette to affect the accuracy of our results.
8. We needed an indicator which has an end point is below pH 7 because there is a sharp change below pH7 and a gradual change above pH 7. We needed a sharp change.
The colours, ranges and end points vary considerably as can be seen in the table below:
(Reference - How far, How fast)
Therefore, we could have used any of the indicators listed in the table above that have a end point below pH7, however due to availability we used methyl red.
The graph below shows a typical strong acid – weak base titration.
This graph clearly shows that there is a sharp change below pH 7 and a gradual change above pH 7.
9. We then used a pipette to accurately measure 25cm3 of limewater, which we then carefully transferred into a conical flask which was placed underneath the burette and drop by drop the hydrochloric acid was added to the limewater. The conical flask was swirled simultaneously on add of hydrochloric acid.
10. On the change in colour of the indicator from colourless to red no more hydrochloric acid was added and the amount of hydrochloric acid required was noted.
This was repeated four times to obtain duplicate results. Repeating my results would ensure that I obtained the most accurate and therefore the most reliable results for this experiment.
Results
Average Volume = 30.60cm3
Number of moles of HCl used = volume × concentration
1000
= 30.60 × 0.02
1000
= 0.0006mol dm-3
The reaction occurring is:
Ca(OH)2 + 2HCl → CaCl2 + H2O
As 2 : 1
HCl Ca(OH)2
0.0006 = 0.0003 moles of Ca(OH)2
2
0.0003 moles in 25cm3
0.0003 × 40
= 0.012 moles
ArCa(OH)2 = 74
Moles = Mass → Mass = Moles × Ar
Ar
= 0.012 × 74
= 0.1g/dm-3
Therefore, the concentration of limewater is 0.1g/dm-3
Safety
Instructions to follow before starting experiment:
- Move all obstructions e.g. stools and bags
- Clear work area of unnecessary equipment
- Beware of spillage’s
- Take care with glass equipment – if breakage occurs report immediately
