Introduction
In this lab, our purpose was to calculate the value of X in the chemical equation CuSO4 ∙ XH2O using our knowledge of percent composition. The only information we were given was that the value of X is somewhere between one and ten which indicates the number of water molecules there are that hydrate the copper (II) sulfate solid.
To solve this problem, our group was to find the mass of the hydrated solid and then compare it to the mass of the anhydrous solid to figure out the percent composition of water in the CuSO4 ∙ XH2O which will eventually lead to the number of moles represented by X in the chemical equation.
Hypothesis
It is predicted that the mass of the hydrated copper (II) sulfate will be greater than the mass of the anhydrous copper (II) sulfate simply because a hydrated substance contains water while the anhydrous substance does not. At this time, it is not possible to predict the number of moles it takes to hydrate the copper (II) sulfate because we do not have the mass of the hydrated and anhydrous substance.
Procedure
Materials
- Hot plate
- Crucible tongs
- Digital balances
- Testing paper
- Crucible
- Hot hands
- Goggles
- Apron
- Sample of CuSO4 ∙ XH2O
- Scoopula
Procedure
- Put on safety equipment (goggles and apron).
- Acquire materials listed in the materials section.
- Using the electronic balance, measure the mass of the crucible. Then set it to zero.
- Place the substance into the crucible and measure the mass of the chemical. Record your observations of the mass.
- Place the crucible with its contents on top of the hot plate.
- Heat the CuSO4 ∙ XH2O on high until there is a visible change in color. Record any observations. Make sure to use the testing paper to identify if water vapor is being released.
- Once the change has occurred, remove the crucible using the hot hands/crucible tongs.
- Turn off the hot plate.
- After the crucible has cooled, check the mass of the substance once again using the electronic balance.
- Clean up workspace and return materials back to original locations.
*Note: The hot plate can burn you. Pay attention to what you are doing. If you burn yourself, immediately run your hands under cold water and inform your teacher of what happened.
Materials used in this lab may shatter if placed under heat. Also, remember not to remove your safety equipment consisting of goggles and an apron until your teacher tells you to do so.
Observations
| Hydrated Copper
(II) Sulfate Appearance | Mass of Hydrated Copper (II) Sulfate | Anhydrous Copper
(II) Sulfate Appearance | Mass of Anhydrous Copper (II) Sulfate |
| The substance was a blue colour. | 1.6g | The substance was a dull, white color. | 1.2g |
Table 1: Observations of the CuSO4 ∙ XH2O were recorded before and after removing the water by heating the substance. The mass of the substances was taken using an electronic balance. To check if water was evaporating when we heated the substance, we used the testing paper to check for water vapor. The paper turned pink indicating that we were dehydrating the substance by releasing the water.
Discussion
Calculations:
| Part of Substance | Mass (g) | Percent Comp. | Molar Mass | Number of Moles | “Cleaning up” | Ratio |
| CuSO4 | 1.2g | 75% | 159.61
g/mol | 0.00752
mol | 1 | 1 |
| H2O | 0.4g | 25% | 18.0153
g/mol | 0.022203
mol | 2.95 | 3 |
| Element | Percent Composition |
| Copper (Cu) | 35.78% |
| Sulfur (S) | 18.05% |
| Oxygen (O) | 45.04% |
| Hydrogen (H) | 1.13% |
Table 2 & 3
To figure out the mass of the CuSO4 and H2O, we looked at the mass of the hydrated and anhydrous versions of copper (II) sulfate and noticed a difference of 0.4 between 1.6 and 1.2 which indicated that the mass of the water was 0.4g while that mass of copper (II) sulfate was 1.2g. To calculate the percent composition, we took the mass of each part of the substance and divided it by the total mass. This concluded that 75% of the substance was copper (II) sulfate while 25% was water.
To calculate the molar mass, we added up each element’s atomic mass for each part of the substance. By doing this, it figured out that the molar mass for copper (II) sulfate is 159.61 g/mol, while the molar mass for water is 18.0153 g/mol. To calculate the number of moles we used the formula triangle. For the two parts of the substance, we took the mass and divided it by the molar mass to find out that there was 0.00725 mol of copper (II) sulfate while there was 0.022203 mol of water.
To “clean up” we divided each of the moles by the lowest number which was 0.022203 (water). This resulted in the ratio of copper (II) sulfate to water being 1:3. In table 3, we calculated the percent composition of each specific element by taking the mass of the element and dividing it by the total mass of the compound, and multiplying it by 100.
