Determining the Water Percentage Within Copper Sulfate Pentahydrate Compound Sample

Hydration Lab

Purpose: The purpose of this lab was to determine the percentage of water within the sample copper sulfate pentahydrate compound. The goal of the students was to record the amount of water evolved in comparison to the amount of copper sulfate and calculate the amount of moles of water within the compound as well as the percent composition of water in copper sulfate pentahydrate.

Hypothesis: If the water is evolved from the copper sulfate, then the amount of moles of water within the copper sulfate pentahydrate compound for both trials will be equivalent because the amount of moles is proportion to the molar mass of the copper sulfate and will result in the same ratio.

Theory: A mole is used to measure the amount of a substance whose number of particles is the same as the number of atoms of carbon in exactly 12 grams of carbon-12. One mole of water equates to 18.02 grams. One mole of copper sulfate equates to 159.61 grams. The mass of water evolved is calculated by taking the difference of the hydrated sample from the dehydrated sample. Both of these samples are calculated by taking the difference of the mass of the crucible, cover, and sample from the mass of the crucible and the cover. The number of moles of water within the copper sulfate pentahydrate is calculated by finding the amount of moles of water and the amount of moles of the unhydrated copper sulfate and dividing the moles of water from the moles of unhydrated copper sulfate. The actual value of moles of water per copper sulfate is 5 moles and the percent composition of water in copper sulfate pentahydrate is 36.1%

Variables:

Independent Variable: Mass of crucible, cover and hydrated sample in grams

Dependent Variable: Mass of water evolved in grams

Controlled Variables: Bunsen burner, balance, scoopula, amount of time, temperature of fire, temperature of room, type of air and quality of air

Materials:

1. Digital balance measured to the 100th place
2. Spatula
3. Bunsen burner
4. Matches
5. Crucible
6. Crucible cover
7. Ring stand
8. Ring
9. Clay triangle
10. Crucible tongs
11. Unspecific amount of copper sulfate pentahydrate
12. Apron
13. Goggles
14. Stopwatch

Procedure:

1. Measured mass of crucible and cover on balance
2. Measured mass of crucible, cover, and hydrated sample on balance
3. Calculated mass of hydrated sample with previous measurements
4. Placed the Bunsen burner under one ring connected to the ring stand
5. Placed the clay triangle over the ring
6. Turned on the gas and light the Bunsen burner with the matches
7. Placed the crucible, cover, and hydrated sample over the fire using the crucible tongs
8. Kept the crucible cover slightly open and made sure fire from the Bunsen burner just barely touches the bottom of the crucible
9. Kept the crucible, cover, and hydrated sample heated in this position for ten minutes
10. Turned off Bunsen burner and waited for five minutes
11. Removed crucible and cover from the clay triangle using the crucible tongs and placed crucible and cover on the balance
12. Recorded the mass of crucible, cover, and dehydrated sample
13. Repeated steps 6-12 with an additional heating period of five minutes and another cooling period of five minutes
14. Repeat steps 1-13 for a second trial

Qualitative Observations:

• Copper sulfate pentahydrate started out as a blue powder
• After heating, copper sulfate pentahydrate turned white on the top layer and yellow on the bottom layer
• Dehydrated copper sulfate turned into a white and yellow brittle powder and was easily broken down with a spatula
• The mixture was not homogenous
• Crucible and cover were white
• Crucible and cover were not affected by the heat
• Crucible and cover extremely hot surfaces after heating
• If water is added to the dehydrated copper sulfate, the compound turns to the same shade of blue and a sizzling sound is heard

Results:

Sample calculations:

Mass of hydrated sample = (mass of crucible, cover and hydrated sample) – (mass of crucible and cover)

Hydrated Copper Sulfate: (20.13 grams) – (18.60 grams) = 1.53 grams

Mass of dehydrated sample = (mass of crucible, cover and dehydrated sample) – (mass of crucible and cover)

Dehydrated Copper Sulfate: (20.13 grams) – (19.66 grams) = 1.06 grams

Mass of water evolved = (mass of hydrated sample) – (mass of dehydrated sample)

Water Evolved: (1.53 grams) – (1.06 grams) = 0.47 grams

Number of moles of water = (mass of water in grams) X (1 mole of water/18.02 grams of water)

Moles of water: (0.47 grams) X (1 mole of water/18.02 grams of water) =

0.026 moles of water

Number of moles of copper sulfate = (mass of copper sulfate in grams) X (1 mole of copper sulfate/159.61 grams of copper sulfate)

Moles of copper sulfate: (1.06 grams) X (1 mole of copper sulfate/18.02 grams of copper sulfate) = 0.0066 moles of copper sulfate

Number of moles of water per 1 mole of copper sulfate: (number of moles of water)/(number of moles of copper sulfate)

Moles of water per 1 mole of copper sulfate:

(0.026 moles of water) / (0.0066 moles of copper sulfate) = 3.93 moles of water per 1 mole of copper sulfate

Trial #1 Trial #2

Mass of crucible and cover 18.60 grams 17.67 grams

Mass of crucible, cover, and hydrated copper sulfate 20.13 grams 19.25 grams

Mass of hydrated copper sulfate 1.53 grams 1.58 grams

Mass of crucible, cover, and dehydrated sample – 1st weighing 19.68 grams 18.62 grams

Mass of crucible, cover, and dehydrated sample – 2nd weighing 19.66 grams 18.62 grams

Mass of dehydrated sample 1.06 grams 0.95 grams

Mass of water evolved 0.47 grams 0.63 grams

Figure 1: Measurements and Calculations of Copper Sulfate Samples

Trial #1 Trial #2

Moles of water evolved 0.026 moles of water 0.035 moles of water

Moles of dehydrated sample 0.0066 moles of copper sulfate 0.0060 moles of copper sulfate

Number of moles of water per one mole of copper sulfate 3.93 moles of water per mole of copper sulfate 5.87 moles of water per mole of copper sulfate

Percentage Error 21.5% 17.5%

Percent Water in Sample 30.7% 39.9%

Figure 2: Calculations of Copper Sulfate Samples

Experimental Average Actual Value Percent Error

Number of moles of water per one mole of copper sulfate 4.90 moles of water per mole of copper sulfate 5 moles of water per mole of copper sulfate 2%

Percent Water in Sample 35.3% 36.1% 2.2%

Figure 3: Comparison of Figure 2 to Actual Value

Figure 4: Moles of water comparison of trials and actual value

Figure 5: Evaporation of water in crucible over the 15-minute period

Figure 6: Mass comparison of samples of copper sulfate pentahydrate and water

Conclusion:

While the relative mass of the water evolved was similar, the small difference in the amount of water evolved created a significant difference in the amount of moles of water per mole of copper sulfate. Both trials followed similar paths as shown in Figure 5. The mass of the crucible, cover, and hydrated sample decreased as the water evaporated from the crucible. Because of the small opening, the water turned into vapor and escaped from the crucible. Both trials were neither precise nor accurate given the percentage error of almost 20% for both trials. However, as shown in Figure 3, the experimental average of both trials created an average that was relatively accurate to the actual value. The experimental value was 4.90 moles of water per mole of copper sulfate, which resulted in only a 2% margin of error. While the number of moles of water per copper sulfate for trial 1 was significantly lower than the actual value, the experimental average was close to the actual value because the number of moles of water per copper sulfate for trial 2 was significantly higher. In Figure 1, not only did trial 2 contain more water evolved, but also less dehydrated sample after the 20-minute period. The 2nd weighing period proved mostly insignificant as most of the water evaporated in the first 10 minutes. For trial 1, the additional 5 minutes under the Bunsen burner resulted in 2 additional grams of water evolved. Moreover, in trial 2, no additional water evaporated in this extra 5 minutes. The results did not support the hypothesis because the amount of moles for the two trials were not remotely close to each other or the actual value. Trial 2 contained 1.94 more moles of water per mole of copper sulfate. Trial 2 contained 50% more moles of water per copper sulfate pentahydrate than trial 1. While the objective of the experiment was met, the results were not congruent with the actual value of 5 moles of water per mole of copper sulfate. The dehydrated copper sulfate was not homogenous because the heat from the Bunsen burner disproportionally affected the lower half of the samples. The copper sulfate was also extremely brittle and powdery in nature as it lost all water during the heating process. The mass of the water was significant as the percent composition for trial 1 was over 30% and the percent composition of trial 2 was almost 40%. This high percent composition of water within the copper sulfate gave the copper sulfate pentahydrate its light blue color.

Evaluation of Procedure:

The main problem with the procedure was the implementation of the Bunsen burner and the crucible cover. The amount of heat given off by the Bunsen burner was not controlled well enough in the experiment. A slight increase in the heat from the burner could have drastically changed the amount of water evolved. In addition, the procedure only noted to keep the crucible cover slightly open and did not specify how open the cover should be. A shift of the crucible cover would change the area where the water vapor could leave the crucible. There was also a limitation with the time constraint as only two 20-minute trials could be tested. More trials could potentially give more precise data and a better experimental average in additional to a bigger database to evaluate. Moreover, despite the fact that both trials lasted for 20 minutes, the trials did not start or stop at the same time. In addition to the procedure, the experimental equipment could also be improved to give more accurate results. The balance only measured to the 100th place in grams and none of the crucibles or crucible covers were the same mass.

Improving the Investigation:

The experiment could be controlled by keeping the mass of the crucible and crucible cover the same. While this was virtually impossible with the current experimental equipment, a controlled crucible and crucible cover would eliminate another variable. A more precise balance could be used to measure the mass of the crucible, cover, and samples of copper sulfate. Additionally, equipment to measure the temperature of the Bunsen burner and control the output of the Bunsen burner would also eliminate experimental bias in the procedure. The spatula was also inefficient in transporting the copper sulfate as it was nearly impossible to keep the mass of the hydrated sample the same for the two trials.