Experimental Report
Data Collection and Processing
Photograph of Lab Setup
Photo 1: Laboratory Setup
- Crucible was used because magnesium could melt plastics when its temperature is high.
- In order to prevent the crucible from falling through the tripod, a clay triangle was employed.
- Bunsen burner was placed underneath the crucible so that the magnesium could be heated efficiently.
Qualitative Observation
Photo 2: Reaction taking place
At the beginning of the reaction, there were not any changes. Meanwhile, the lid was lifted twice. A few minutes later, the magnesium started glowing brilliantly (see the photo above). It glowed for a while until it stopped naturally. White powder was seen in the crucible.
Raw Data Table
*Ms. Crook’s class’ data was used because the data obtained in Table 1 did not have enough dots to draw a graph.
Data Processing
Overview
In order to calculate the empirical formula of magnesium oxide, the net masses of magnesium, magnesium oxide and oxygen were first to be determined. To reach molar ratios, the number of moles of magnesium and oxygen were reached and simplified into the lowest whole number ratios. The empirical formula of magnesium oxide was then decided. During data processing, the percentage uncertainties associated with masses were found so that the percentage uncertainties of the number of moles were concluded. They were later converted into absolute uncertainties. A scatter graph was drawn to show the compositions of magnesium and oxygen within magnesium oxide. The percentage yield of magnesium oxide was also found.
Sample Calculation
Presentation
*The percentage uncertainties of the number of moles were the percentage uncertainties of the mass of each element (explained in Sample Calculation Section). The final value was the greatest percentage errors in Table 2.
*The values of some of the absolute uncertainties are 0.000. This does not mean that there were no uncertainties; it was only because the uncertainties were so small that when they were rounded up to 3 decimal places, they became 0.000.
*Trial 4 seemed to be anomalous. Thus, when calculating the average, the results from Trial 4 were excluded. It is noticed that Ave. of n (Mg) is 0.006 and Ave. of n (O) is 0.004; their ratio seems to be 3:2. However, all the calculations were done by Excel and rounded up to 3 decimals. As a result, the Ave. ratio calculated is 1:1.
*Extrapolation was involved drawing the graph (magnesium: 0.000 mol, oxygen: 0.000 mol) since it is believed when there is no magnesium present, no oxygen will be reacting to form magnesium oxide.
This graph shows the relationship of the composition of magnesium and oxygen within the compound, magnesium oxide. It can be seen that the amount of magnesium is proportional to the amount of oxygen. The gradient of the line of best fit is 0.6005. The positive gradient indicates that an increase in the amount of magnesium will result in a proportional increase in the amount of oxygen. There are outlier points in the graph. The most obvious one could have affected the value of the gradient. In addition, the correlation coefficient (R2) is 0.5751, which is away from 1, indicating not all the points lie on the linear line.
This graph presents the same information as the graph shown above except that it excludes the most obvious outlier point (0.006, 0.001). As a result, the gradient increased (became 0.694) and the R2 value got closer to 1.
Conclusion and Evaluation
Conclusion and Justification
According to the processed data, the hypothesis that the combustion of magnesium will generate data which can be used to calculate the empirical formula of magnesium oxide is supported because the molar ratios of magnesium and oxygen were managed. As it is shown in Table 3.1, the average ratio of magnesium and oxygen is 1:1, suggesting the empirical formula of magnesium oxide is MgO. However, it has to be noted that some of the results gained were controversial to each other, i.e., in one particular case, the calculated empirical formula of magnesium oxide is Mg4O. From Table 3 and Table 3.1, the possible empirical formulas for magnesium oxygen were: MgO, Mg2O or Mg4O.
Theoretically, the empirical formula for magnesium oxide should be MgO because MgO is an ionic compound. When magnesium atoms and oxygen atoms react with each other, magnesium atoms lose 2 electrons to become Mg2+ (magnesium ions have a valence of 2+ charge) and oxygen ions gain 2 electrons to become O2- (oxygen ions have a valence of 2- charge). The outer shells of both ions will then be full so that they become stable like the noble gases. That is to say, the molar ratio of magnesium and oxygen should be 1:1. Thus, the gradient of the scatter graph ought to be 1; though in the first graph the gradient is 0.6. After the elimination of the anomalous, the gradient became 0.7 which is closer to 1.0. It can be deduced that the off-true-value gradient was caused by some outlier points. From both Table 3.1 and Graph 1, it can be seen that the absolute uncertainties of the mass and the number of moles of oxygen were extremely large, indicating the unreliability of the experiment results. Comparing to Graph 1, the correlation coefficient in Graph 2 improved after the outlier point was excluded, i.e., it became 0.9, demonstrating that most of the dots lie on the linear line whose gradient is 0.7. In addition, the percentage yield in Table 4.1 shows the completion of the reaction. None of the results were close to 100%, suggesting magnesium did not react completely. Thus, the empirical formulas obtained were not accurate enough to determine the true formula.
In conclusion, the empirical formula of magnesium oxide can be calculated by the known; yet the accuracy may be affected by circumstances, such as the completion of the reaction. Nevertheless, the theoretical empirical formula of magnesium oxide is MgO.
Limitations of Experimental Design
Generally, the experiment went well since both qualitative and quantitative data had been produced. The empirical formula of magnesium oxide was deduced. Nonetheless, improvement can be made to achieve better results.
There were anomalies present (see Table 3 and Table 3.1): one of the molar ratios calculated was 4:1; some others were 2:1. They were considered outlier points because they are off the true value which is 1:1. The possible reasons could be: 1) the reactions were not fully completed; 2) the readings of the masses were incorrect since it took a while for the electronic balance to reach the real mass. Thus, if the experimenter did not wait till the readings stopped changing; the recorded data would be smaller than they were supposed to be. These could cause random errors. Another random error could be that when the crucible lid was lifted, the produced powdered magnesium oxide might have escaped the crucible. The mass would then be heavier than it was weighed. As shown on the graph, R2 (the correlation coefficient) equals to 0.5751, indicating that the data was not very reliable since the value was away from 1. In addition, the data points did not spread out, i.e., they were gathering at 0.004 to 0.006 moles. Consequently, the gradient was influenced. The error bars were obviously significant, suggesting the recorded results were not precise.
Systematic errors could be caused by the precision of the equipment, e.g. the electronic balance. However, they can be avoided (see the next section).
Suggestions for Improvement
