Stoichiometry and empirical formula

The composition of hydrated crystals is obtained by weighing before and after heating.

X, the ratio of moles of water for each mole of copper sulfate, is determined through calculation. It is assumed that when hydrated crystals are heated only the water in the crystal structure is driven off and that no decomposition has occurred.Mass of anhydrous copper sulfate= 28.780-28.190= 0.

590g ±0.002 No. of moles of anhydrous copper sulfate= 0.590/(63.

55+32.06+4(16))= 0.0036965mol ± 0.33898%= 0.00370mol ±0.00001 Mass of water= 29.

203-28.780= 0.423g ±0.002No. of moles of water= 0.423/(2(1.

01)+16)= 0.02347391787mol ±0.00000846%= 0.023473918mol ±0.000000002 X= 0.0036965/0.

02347= 6.349± 0.33898846%= 6.35 ± 0.

02% precision Uncertainty= 0.339%% Error= (6.349-5)/5×100= 27.0%Av. % precision uncertainty of class data= 0.

317%% Error= (5.59208-5)/5×100= 11.8%% uncertainty of mean ratio= 7.96%Since % uncertainty of mean ratio is higher than the av. % uncertainty, 7.

96% is used instead.The class results are more accurate (5.59) compared to my results (6.35). The class results are not consistent as the range (3.

88) and % uncertainty (7.96%) is large. Both readings are precise as the % precision errors are low. The % error (27.0%) of my readings is larger than my % precision uncertainty (0.339%) readings.

The % error (11.8%) of the class readings is larger than their % precision uncertainty (7.96%) readings. Thus, systematic and random errors are present in both cases. Most of the class data is noted to be higher than the literature value (5).The errors that may have caused my and most of the class’s x to be higher are: 1.

(Systematic error) The crystals were observed to not flow but stick to each other. The copper sulfate was wet and had more water of crystallization. This is because water may have been absorbed from the atmosphere. Thus the mass of water present in the copper sulfate was more than expected, causing x to be higher. 2.

(Random error) If white fumes escaping from the crucible were observed, the copper sulfate had been lost during heating. Thus the mass of copper sulfate present decreased, causing x to be higher than expected. 3. (Random error) During heating, if black solids are observed to be formed, the crystals of copper sulfate may have decomposed e.g.

to copper oxide. This would cause the mass of copper sulfate to be lesser than expected and thus, x would be higher.Some of the class data was lower than expected. The errors that may have caused x to be lower are: 1. (Random error) The copper sulfate was not heated long enough. The heating was incomplete and thus the measured mass after heating would be higher than expected, causing x to be smaller.

2. (Random error) On cooling, the copper sulfate could have absorbed water from the surroundings. Thus the measured mass after heating would be higher than expected, causing x to be smaller.The measurements taken are very precise as the class used the same apparatus and reagents.

However, the readings taken are inaccurate due to the copper sulfate having more water of crystallization. The correct method of heating the copper sulfate should also be carried out as students either heated the copper too strongly, too long or short.Suggestions: 1. Re-heat the crystals on cooling a few times to ensure no water of crystallization is left. This would improve accuracy of readings. 2.

Heat the crystals on a gentle flame. This would improve consistency. 3. Keep the lid of the crucible on to ensure that the copper sulfate does not escape as a vapour. This would improve accuracy. 4.

Place copper sulfate in a desiccator before heating and during cooling to make sure it doesn't absorb moisture from air increasing its weight. This would improve accuracy. 5. Repeat the experiment a few times and find the average readings.

This would reduce random uncertainty and increase accuracy.