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Moles of Ca(OH)2 = the number of moles of HCl / 2 (as the ratio is 2 : 1) = 0.0008470 / 2 = 0.0004235 mol (4sf)
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Moles of Ca(OH)2 present in 100cm3 of alkaline solution = 0.0004235 / 25 = 0.0000169 * 100 = 0.001690 mol (4sf)
- Relative atomic mass of calcium = mass / moles
= 0.10 / 0.001690 = 59.171597
= 59.17 (4sf)
Evaluation
The results gained by the two methods show that both of the methods are not very accurate. This can be seen when the percentage error is calculated, for method 1:
Percentage error = (64.85 – 40.1) / 40.1 * 100 = 61.72 % (4sf)
This high percentage error could have been caused because of the following reasons.
As the calcium was added to the water I had to put the bung onto the flask as soon as possible. It is not physically possible to put the bung on instantly after the calcium is added to the water. Therefore in this short time period between the addition of calcium and the bung being put on, a certain amount of gas would escape into the atmosphere and not into the measuring cylinder. Because of this, when the relative atomic mass of calcium is being calculated, it would come out higher.
The measuring cylinder measured to the 1 cm3. This means there is a possibility for +1 cm3 error or –1 cm3 error. If a positive 1 cm3 error had been made then the relative atomic mass calculated would seem higher and the vice versa for a negative 1 cm3 error. A gas syringe would reduce the possibility or error as it measures to 0.5 cm3 . If the gas syringe is used, it would give a percentage error of 1.35 % (0.5 / 37 * 100 = 1.3513). Where as the measuring cylinder gave a percentage error of 2.70 % (1 / 37 * 100 = 2.7027). This shows if the gas syringe is used it would reduce the gap for error.
Another possible error is that the calcium could have been kept for so long that it might have oxidised when it was left out in the open. This would mean that less calcium would react and would have lowered the value of the gas produced. This would give a higher relative atomic mass when calculated. To prevent this, if the experiment is repeated, it can be made sure that the lid on the pot of calcium is always kept on and only taken off when it is needed. This would reduce the oxidising of the calcium.
The reaction may have been stopped before it was completed. This would results in less gas being produced and this would make the relative atomic mass higher. Also there could have been a default in the equipment being used. There could have been leakage in the pipe, causing a certain amount of gas to escape into the atmosphere and not into the measuring cylinder. This would result in a higher relative atomic mass.
The accuracy of this method can be increased by:
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Using a gas syringe to collect and measure the gas instead of a measuring cylinder. This would mean the accuracy would increase by 0.5 cm3 as the measuring cylinder measures to 1 cm3 and the gas syringe measure to 0.5 cm3.
- When measuring the calcium, a more accurate weighing scale could be used. The weighing scale used in this experiment measured to 2 decimal places (0.01), which is fairly accurate but this could be increased by using a weighing scale which measures to 3 decimal places.
- This method was only carried out once. To gain a respectable average and also to see if any anomalous results are gained, the method can be carried out at least 3 times.
Method 2:
This method was also not accurate as shown by the percentage error gained.
Percentage error = (59.17 – 40.1) / 40.1 * 100 = 49.56 % (4sf)
The actual molar concentration of the hydrochloric acid may have been different to 0.1 mol. The acid may have been diluted as all of the students were using it. If the acid was diluted than the relative atomic mass gained would have been higher.
The first test performed in this method gave the value of 9 cm3, this was not closely in the same region as the other two values gained, which were both 8.20 cm3 . I had still calculated my average value considering the 9 cm3 . I chose not to eliminate this value in the calculation of the average because in my opinion it was not too far away from the region of the other two values. If I had considered this value as an anomaly. The explanation for this could be:
- Too much hydrochloric acid could have been allowed into the solution, as it was quite difficult to have full control over the speed that the hydrochloric acid was being let into the solution. If too much hydrochloric acid had been let into the solution, the value recorded in the results would be higher therefore the average would come out higher and this would give a lower relative atomic mass.
The weighing scale used measured to 2 decimal places. To make the results more accurate and reduce the percentage of error a weighing scale which measured to 3 decimal places could have been used. This aspect has already been discussed in the method 1 part of the evaluation.
The experiment could also be carried out a higher number of times to get an more accurate average and more accurate relative atomic mass.
The error with the oxidisation of the calcium could also apply to this method and has been discussed in the first part of the evaluation.
In comparison method 2 is more accurate than method 1. This is because method 2 gave a relative atomic mass of 59.17 which is closer to the factual value of 40.1when compared to the value gained by method 1, 64.85. Also the accuracy can be linked to the percentage error as method one gave a higher percentage error than method two (method 1 = 61.72 %, method 2 = 49.56 %).
Overall, both methods are very inaccurate (high percentage errors). But in comparison, method 2 is a better way of determining the relative atomic mass of calcium.
