A student used 5.06g of copper sulphate pentahydrate and salicyclic acid in the performance of the experiment. The student recovered 3.094g of salicyclic acid and 2.146g of copper sulphate pentahydrate. What is wrong with the student's result? How might this error have been avoided?
The student's results are incorrect because the sum of the recovered salicylic acid and copper sulphate pentahydrate (3.094g + 2.146g = 5.24g) is greater than the initial mass used in the experiment (5.06g). This suggests that there was some contamination or error during the experiment.
This error could have been avoided by taking the following precautions:
1. Ensuring the weighing balance is properly calibrated and accurate.
2. Using clean and dry containers for weighing and transferring the chemicals.
3. Making sure not to mix any other chemicals or substances which could lead to the observed discrepancy.
4. Recording the measurements carefully and avoiding any errors in calculation.
5. Repeating the experiment to confirm the reproducibility of the results.
The student's result indicates that they recovered more salicylic acid than they initially used, which is not possible in a chemical reaction. Additionally, the student obtained less copper sulfate pentahydrate than they started with, which suggests incomplete recovery.
This error might have occurred due to incomplete drying of the recovered substances. Copper sulfate pentahydrate contains water molecules, and if the student did not properly dry it, the remaining water could contribute to the weight difference. Similarly, if the salicylic acid was not completely dried, it could also retain some moisture, leading to an inaccurate measurement.
To avoid this error, the student should have ensured thorough drying of both the salicylic acid and copper sulfate pentahydrate before weighing them. This can be achieved by heating them gently to remove any remaining water. By doing so, the student would obtain more accurate weights and avoid this discrepancy in the experimental results.
To determine what is wrong with the student's result, we need to compare the experimental data with the expected theoretical values. Let's calculate the theoretical values first.
1. Calculate the molar mass of copper sulfate pentahydrate (CuSO4·5H2O):
- Molar mass of CuSO4: 63.55 g/mol (copper: 63.55 g/mol, sulfur: 32.07 g/mol, oxygen: 16.00 g/mol, 4 oxygens in the formula)
- Molar mass of 5H2O: 90.10 g/mol (5 moles of water, each with a molar mass of 18.02 g/mol)
- Total molar mass = 249.68 g/mol (CuSO4: 159.61 g/mol + 5H2O: 90.10 g/mol)
2. Calculate the expected mass of copper sulfate pentahydrate (CuSO4·5H2O) using the molar mass from step 1:
- Mass = moles * molar mass
- Moles of CuSO4·5H2O = mass / molar mass = 2.146 g / 249.68 g/mol
Now, let's calculate the expected mass of salicylic acid (C7H6O3):
1. Calculate the molar mass of salicylic acid (C7H6O3):
- Molar mass of C7H6O3 = 138.12 g/mol (carbon: 12.01 g/mol, hydrogen: 1.01 g/mol, oxygen: 16.00 g/mol, 7 carbons, 6 hydrogens, 3 oxygens)
2. Calculate the expected mass of salicylic acid using the molar mass from step 1:
- Mass = moles * molar mass
- Moles of C7H6O3 = mass / molar mass = 3.094 g / 138.12 g/mol
Now let's compare the calculated theoretical values with the experimental data:
- The expected mass of CuSO4·5H2O is obtained by multiplying the moles of CuSO4·5H2O by its molar mass (2.146 g / 249.68 g/mol). Compare this with the experimental mass of 2.146 g. If they are not equal, it indicates a difference between the expected and experimental values.
- The expected mass of salicylic acid is obtained by multiplying the moles of salicylic acid by its molar mass (3.094 g / 138.12 g/mol). Compare this with the experimental mass of 3.094 g. If they are not equal, it indicates a difference between the expected and experimental values.
Based on the calculations, we can determine what might be wrong and how the error could have been avoided:
1. Theoretical calculations: Validate that the theoretical calculations done to determine the expected masses are correct. Check the molar masses, the stoichiometry of the reaction, and the calculations for determining the theoretical masses.
2. Experimental techniques: Ensure that the measurements and procedures followed by the student during the experiment were accurate. Check for any potential sources of error, such as spillage, incomplete reactions, or losses during the process of recovering the substances.
To avoid such errors:
- Be careful and precise when measuring substances during the experiment.
- Use analytical balances with a higher precision to get more accurate mass measurements.
- Refine the experimental technique by repeating multiple trials to account for any unforeseen variations and improve the accuracy of the results.
By following these precautions, the student can minimize errors and improve the accuracy of their experimental results.