Hydration Lab

Data Processing

How many moles of water were there in the sample?

0.44 ± 0.11 g H2O  ·    1 mol H2O   =

                           18.02g H2O

How many moles of Copper[II] Sulfate were there in the sample?

0.96 ± 0.11 g CuSO4  ·    1 mol CuSO4   =

                             159.60g  CuSO4

Based on this data, what is the water of hydration?

   0.024 ± 0.006 g  H2O      =      0.24 ± 25% g H2O       =     4 ± 25% g H2O  

0.006 ± 0.0007 g  CuSO4       0.006 ± 12% g CuSO4        1 ± 12% g CuSOB

∴ There are 4 water molecules for each molecule of CuSO4, with an error of 37%. This means that the value could be off by more than one molecule (4 · ±37% =  ±1.48)

What would the chemical formula and name for the compound be?

Formula:          CuSO4 • 4H2O  _        

Name:              Copper[II] Sulfate Tetrahydrate        

What is the literature value for this?

Formula:          CuSO4 • 5H2O   _                 _        

Name:              Copper[II] Sulfate Pentahydrate        

What is the Percentage Deviation (error)?

%Deviation = |(4-5)÷5|×100% = |-1÷5|×100% = 0.2×100% =

Data Presentation

Figure 1. Graph of sample broken into proportions by mass of water and copper[II] sulfate

Conclusion

        The purpose of this lab is to determine the water of hydration of a compound (CuSO4) by heating it and hence find its chemical formula. It was determined that the compound contained 4 moles of H2O for every 1 mole of CuSO4, hence had the formula  CuSO4 • 4H2O and Copper [II] Sulfate Tetrahydrate. However, the literature value was 5 moles of oxygen for every  1 mole of CuSO4 (Copper [II] Sulfate Pentahydrate - CuSO4 • 5H2O).

        As calculated above in Data Processing, this means that the percentage deviation from the literature value was 20%. This is significantly less than the percentage uncertainty, which was 37%. This should indicate that the random error was higher than systematic error, because while there was a high level of uncertainty, the literature value still falls within that value, meaning that the measurements were close enough.

        In reality, though, this high level of uncertainty says more about the inaccuracy of the measuring equipment than it does about the amount of systematic error. For example, it is possible that simply not all of the water was removed from the compound. On the contrary, the lab procedure instructed to continue trials of heating the compound and weighing it until the mass values were within 0.05g of each other. Those in this lab were only 0.01g apart – a difference which would not have affected the outcome of the lab. This means it is unlikely that there was a significant amount of water remaining in the compound.

Evaluation of Procedure

        It cannot honestly be said that this lab was executed successfully. This is for two main reasons:

1. the false result
2. the high degree of uncertainty

        The first of these is surely the most important: this lab did not achieve the desired result. While the compound had a water of hydration of 5, the lab results said that this value should be 4. In some circumstances, such error could be tolerated – indeed, 20% is not terrible. However, the very purpose of the lab was to determine the compound, and this goal was not achieved.

        The second point is how uncertain the data really was. Based on the percentage of uncertainty, the water of hydration could have been anywhere from 2.5 to 5.5. This does contain the literature value, but has a range of more than half of  that value (3/5). If the water of hydration could nearly have been 2 or 6, does this lab truly reveal anything? It gives a vague estimate of how much water there is, but little more.

        Much of the uncertainty was caused by an imprecise scale used in the initial measurements. Only after the procedure requested precision to two decimal places was a more precise scale used, and the initial measurements were still only accurate to 0.1g. Also, if the mass of compound used had been higher, the percentage error would have been less simply because it would be a smaller portion of the measured value.

Improving the Investigation

        This lab did not meet its goal, hence definitely should be improved. The simplest change that could be made is to use the more precise scale from the beginning of the experiment. This would reduce the percentage error down to 6.7%, a much more reasonable value. This, however, does not solve the problem of the inaccurate value determined, because it is unlikely that this scale would have been any more accurate, despite being more precise.

        It is hard to pinpoint, therefore, exactly where the experiment went wrong, as it seems that the scales were not at fault but that a sufficient amount of water had been removed. There was certainly some kind of systematic error.

        One way that this error might be reduced, even without understanding its source, is to increase the amount of substance measured. Because the lab is about obtaining a ratio, this would not affect the result at all except hopefully to decrease the effect of any systematic error. This would, however, have the disadvantage of making the lab take longer, and steps might need to be added to ensure that all of the compound has a chance to dehydrate.