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Analysis and evaluation exercise
The experiment was carried out using 0.188 grams of potassium chlorate(V). The volume of gas collected was originally 72 cm3, but on cooling 56.5 cm3 was collected. Work out which is the correct equation for the reaction. Carry out an error analysis on the experiment and suggest possible improvements.
KClO3 = 122.5
\ moles of KClO3 used = 0.188/122.5 = 0.00153
Volume of oxygen at RTP = 56.5 cm3
\ moles of O2 = 56.5/24000 = 0.00235
\ moles of O2 per mole of KClO3 = 0.00235/0.00153 = 1.53
This indicates that the second equation is the correct one:
|
KClO3(s) ⇒ KCl + 1½O2(g) |
Error analysis
The error in the balance is ± 0.002 g as we weighed by difference involving two weighings. This gives a percentage error of 0.002/0.188 × 100 = ± 1.1%.
The gas syringe markings are quite accurate, but most syringes have a degree of stickiness. This allows the plunger to be moved in and out by 2 or 3 cm3, with the correct reading probably being somewhere near the middle of this movement. Given this, it is unlikely that the syringe is accurate to more than ± 1 cm3. This gives a percentage error of 1/56.5 = ± 1.8%.
We have assumed that the apparatus is at room temperature and pressure (we could, of course, always have measured the temperature and pressure!). However, it is unlikely that either is far out, so this should only introduce a very small error.
This gives a total measurable error of ± 2.9%. This means that our answer of 1.53 could actually lie between 1.49 and 1.57. This includes the accepted answer of 1.5.
A larger answer could have been produced if there was any contamination of the sample with the more oxygen rich potassium chlorate(VII), KClO4.
Improvements
Only one result is given. The first obvious improvement is to repeat the experiment to ensure consistent results.
A more accurate balance (± 0.0001 g) would reduce the weighing error, but is unlikely to be available in school. We could also reduce the weighing error with a larger mass, but this would require a larger gas syringe (not readily available) for the volume measurement. A more accurate method of volume measurement would reduce the 1.8% error.
We could have used AnalaR grade potassium chlorate(V) rather than the GPR reagent .