Lab 2: Determine the Percentage of Water in a Hydrate:
The goal of this experiment is to learn how to properly calculate the ratio of salt to water, in a hydrated salt, and to calculate the percentage of water (by mass) within a hydrated salt. By heating a known mass of hydrated salt, evaporating the water (essentially distillation), and then comparing the mass lost to the mass of anhydrous salt left behind, we can calculate the percentage of water within the hydrated salt. We can also (via stochiometric and molar ratios), calculate the ratio of salt to water.
Given that water has a much lower boiling point than copper sulfate, I hypothesize that we can heat it to remove the water, and then calculate the mass that was lost, based on measurements made before and after the water was evaporated. From this, we can calculate the ratio of salt to water (by calculating the molar and stoichiometric ratios), and the percentage of water (by mass) within the hydrated salt (by dividing the mass of the water, by the mass of the hydrated salt, then multiplying the resulting decimal by 100).
Equipment and Materials:
- Crucible with lid
- Clay Triangle
- Bunsen Burner
- Ring Stand
- Mesh Pad, with Asbestos
- Copper Sulfide
- Deionized Water
- Obtained crucible and lid (henceforth, the two are considered to be together unless mentioned otherwise), inspected them, and measured their mass: 36.1574g
- Obtained equipment, set up ring stand, Bunsen Burner, clay triangle, and mesh pad. Ignited Bunsen Burner, and heated crucible for 12 minutes. Placed crucible on mesh pad using tongs (from this point, the crucible was handled solely with tongs until the experiment concluded), and allowed to cool to room temperature. Measured mass of crucible again: 36.1571g
- Measured mass of (hydrous) copper sulfate: 2.1614g
- Measured mass of crucible with (hydrous) copper sulfate: 38.3189g
- Heated crucible with hydrated copper sulfate for 3 minutes, before increasing heat, and heating it for 5 more minutes. At that time, the copper sulfate had turned a yellowish-white. Removed crucible from burner, placed on mesh pad, and allowed to return to room temperature. Measured mass of crucible with anhydrous copper sulfate: 37.3005g
- Added deionized water to the anhydrous copper sulfate, at which point it became hydrated again, and returned to its original blue coloration. Disposed of salt within the proper receptacle, cleaned crucible, and returned all equipment to its point of origin.
Determined mass of anhydrous salt by subtracting the mass of the crucible and lid, from the mass of the crucible, lid, and anhydrous salt: 1.1434g
Determined mass of the water lost by subtracting the mass of the anhydrous salt, from the hydrous salt: 1.018g
Calculated the percentage of water in the hydrated salt:
Determined amount of anhydrous salt by dividing the mass of the copper sulfide by the number of grams per mole, in 1 mole of copper sulfide: 0.01058mol
Determined amount of water lost, by dividing the mass of the water lost during heating, by the number of grams per mole in 1 mole of water: 0.0565mol
Calculated the molar ratio:
Rounded to the nearest integer, the ratio is 1:5.
This means that the formula for hydrated copper sulfate is:
The goal of this experiment is to determine the percentage of water (by mass) in a hydrate, and to calculate the ratio of salt to water in a hydrated salt. To achieve this, a known mass of hydrated salt was heated, evaporating the water (essentially distillation). Given that the mass of the hydrated salt is known, it is also given that the mass lost is equivalent to the mass of the water. By comparing the mass lost to the mass of anhydrous salt left behind, we can calculate the percentage of water within the hydrated salt. We can also (via stochiometric and molar ratios), calculate the ratio of salt to water.
Given the data presented above, findings showed the experimental percentage of water within the hydrated salt to be: 47.10%
The accepted percentage of water within hydrated copper sulfate is: 36.07%
As such, the percent error within this measurement is 30.57%
The experimental stoichiometric ratio between copper sulfate and water was found to be: 1:5.314
The accepted stoichiometric ratio between copper sulfate and water is: 1:5
The percent error within this measurement is 5.909%
The formula for hydrated copper sulfate is:
Possible reasons for such error:
- The anhydrous salt could have been exposed to air prior to measurement, and reabsorbed some moisture, thus disrupting measurements. (Unlikely, considering that the ratios between the two, both stoichiometric and percent water content, would be smaller in such an instance.)
- Not all of the water may have evaporated during heating (Also unlikely, for the same reason as the one above.)
- During heating, some of the hydrous salt may have spattered, thus removing a portion of the hydrous salt from the crucible (A more likely source of error, considering that this would result in a greater difference between the two ratios, and percentages.)
- Human error is always in effect, given that the laboratory does not function under ideal conditions. As such, there is always the possibility of inaccuracies with measurement, perception of measurement, inaccuracies of equipment, and other such errors. (However, this is not likely to be the sole cause of the inaccuracies within this experiment, though it may contribute to it.)
Possible improvements that could be made to this experiment in the future could include increasing the sample size, to produce a more average measurement. Another possibility would be to get more accurate equipment, to replicate the experiment within a dehumidified environment, and/or perform the experiment using a larger crucible (to reduce the possibility that spatter from the hydrated salt could leave the crucible).