Errors in procedure and measurement
- Some soot was formed at the bottom of the crucible. In turn this adds to the total mass, thus preventing me from obtaining the mass purely of the copper sulphate and crucible + lid.
- After the hydrated crystals were heated to become anhydrous crystals, they were left to cool. During this time, water in the air could have reacted with the anhydrous crystals to become hydrous again. This would add onto the mass and cause inaccuracies, thus making the experiment less reliable.
- A lot of thermal energy is required to remove all of the water molecules, thus making it an endothermic reaction. Although the mass remained constant at 2 d.p, it may have been because not enough thermal energy was available to remove all of the water. This adds onto the mass and therefore the reliability of the experiment is hindered. This is therefore an error in both measurement and procedure.
- The conical flask was constantly being swirled whilst being titrated so that the reaction could happen quicker. However different people swirled the mixture, so the rate of reaction may have varied, and so the colour change may have occurred quicker in some experiments.
- Colour change was measured by the eye, therefore the reaction time of different people measuring it varied. The varied time taken to stop the actual titration was due to human reaction time. Thus resulting in human error which reduced the reliability of the results.
- During the titration it was required to stop once the mixture changed colour to pale yellow. However there was no guide or scale as to how pale or to what shade the colour should be. Therefore my judgement could be considered inaccurate. This caused an error in measurement because it was the amount of sodium thiosulphate which needed to be measured.
- Human error caused the reading of the bottom of the meniscus to be possibly wrong, as it proved difficult to find the exact bottom point of it.
- During titration the point at which to discontinue any further titration was difficult because:
- It was too time consuming to add little amounts of sodium thiosulphate and swirl.
- Therefore the end point was difficult to find as it was very easy to accidentally add more thiosulphate than that required because the thiosulphate went into the conical flask relatively fast and in quite large amounts.
Accuracy and reliability
Total % error in experiment 1:
For crucible + lid
+/- [(0.5 x 0.01)/20.82] x 100 = +/- 0.024015369 %
For hydrated crystals
+/- [(0.5 x 0.01)/2.54] x 100 = +/- 0.196850393 %
For anhydrous crystals, crucible + lid
+/- 100 x [(0.5 x 0.01)/22.41] = +/- 0.022311468 %
(Combined) = +/- 0.24317723 %
= +/- 0.2432 (4sf)
Total % error in experiment 2:
Burette:
+/- 100 x [(0.5 x 0.1)/ 23.5] = +/- 0.212765957%
+/- 100 x [(0.5 x 0.1)/ 25.0] = +/- 0.200000000%
+/- 100 x [(0.5 x 0.1)/ 24.1] = +/- 0.207468879%
Mass of crystals:
+/- 100 x [(0.5 x 0.01)/5.83] = +/- 0.08576329331 %
Pipette:
+/- 100 x [(0.5 x 0.06)/25] = +/- 0.12 %
+/- 100 x [(0.5 x 0.06)/25] = +/- 0.12 %
+/- 100 x [(0.5 x 0.06)/25] = +/- 0.12 %
(Combined) = +/- 1.065998129
= +/- 1.066% (4.s.f)
In conclusion it appears that in both experiments 1 and 2, the percentage errors are relatively small which shows that my experiments were fairly reliable and accurate.
The values for % error give a clear indication as to which experiment was more accurate, its apparent that experiment 1 was a lot more accurate than experiment 2 purely because its percentage error was +/- 0.2432 % which is sufficiently less than that of experiment 2 with +/- 1.066 % error.
In experiment 1 the weighing scale gave values to 2 decimal places; however the burette was correct to only 1 decimal place. This reflected in the results as an anomaly was found in Experiment 2.
Furthermore the weight of the crystals remained constant after 10 minutes in experiment 1. But the scale was correct to only 2 decimal places therefore it could suggest that the results obtained in exp. 1 were not entirely correct; because the change to the crystals would have been further than the 2 decimal place.
In conjunction the % error was +/- 0.2432 which is fairly small, henceforth the value could have been 0.2432 % more or less to the result I obtained in experiment 1. As this is a small percentage change, it suggests my value obtained was fairly accurate.
The percentage error in experiment 2 was significantly higher than that in exp. 1. This is highlighted in the results, particularly as the anomaly was found. This suggests that the result I obtained is off by +/- 1.066 % from the actual result. Furthermore because of this % error, it suggests that the results I obtained are not as accurate as they could be.
In conclusion by obtaining the correct ratio for the formula suggests my experiment was fairly accurate. Furthermore the % errors were not large enough to hinder my experiment greatly.
Improvements for errors in measurement and procedure
- The temperature at which the crystals fully turn anhydrous should be looked up and this temperature should be monitored so that there is no chance that any hydrated crystals are left in the crucible, as this would add on to the weight.
- The top pan balance used should be correct to more decimal places. Although the weight of the crystals remained constant, it could have been a matter of further mass change to more than just 2 decimal places.
- The crystals should be crushed into even smaller pieces so that the surface area is large and there is a greater chance that every single crystal has turned fully anhydrous.
- The crystals should be heated in pure oxygen to avoid incomplete combustion which is the root cause of soot formation. The soot adds to the mass therefore it is vital to avoid any incomplete combustion as it will cause inaccuracies.
- During titration less amount of solution should be added in order to find a more accurate end point. In addition more time should be spent whilst swirling the conical flask.
- It was difficult to judge the end point in experiment 2, because uncertainty is caused as to when to stop. To eradicate human error in this case, a colorimeter should be used to give a numerical value of when to stop the titration.
Most significant error in procedure and modification
- In experiment 1, the most significant error is the possibility of not all the crystals becoming anhydrous. This is because the temperature at which this happens is unknown, and because the reaction was highly endothermic, a lot of heat is required to fully make the crystals become anhydrous. Therefore it is not known if a Bunsen burner can provide enough thermal energy which made all the water evaporate. Heating it too much however causes the copper sulphate to decompose, thus also hindering the results of the experiment. If the water is not fully driven off, it will create an error in measurement because the mass of the copper sulphate will not be correct. The modification for this is that the temperature at which the copper sulphate loses all of its water molecules needs to be researched before hand. This provides the precise temperature at which the copper sulphate needs to be heated, in order for all of the water to be evaporated, whilst avoiding the decomposition of the copper sulphate. This will ensure that the measurement for the mass of the crystals is accurately known.
- In experiment 2, the most significant error is knowing the exact point as to when to stop the titration. Due to human error, one’s perception of when the mixture has changed its colour can vary to another person. Therefore the incorrect volume of thiosulphate may be measured, thus causing the results to become less reliable. In order to eradicate this error, a colorimeter should be introduced which provides a numerical measurement of when to stop the titration. This takes away the chance of the human error, thus giving more accurate results, hence further making the experiment more reliable.
Most significant error in measurement and modification
- From both experiments, the apparatus with the least % accuracy was the burette. It had errors up to +/- 0.212765957% which is the largest error out of all the equipment. This error can be minimised by using a burette with a smaller diameter. This allows the meniscus to be seen easily compared to that with a larger diameter. By doing so a more accurate result is gained, hence a more accurate reading of the volume of solution in the burette is obtained and so the results are more reliable.
- Another modification is to use a burette with a much smaller scale. by using a scale which is more accurate than 0.1ml would allow for a more accurate reading of volume during titration. This effectively would allow for more accurate calculations and therefore results of higher accuracy.
- A burette which has a much narrower tip should be used. This is allows fewer drops to be added in a given time, in effect colour changes can be spotted more accurately thus preventing an excess amount (which is not required) added. This prevention allows for more accurate results, therefore a more accurate and reliable experiment.