Chemistry Lab - Determining A Molecular Formula


Purpose: To determine the molecular formula of CuSO4 • xH2O using the mass of percent of water.

Hypothesis: The variable x will be 5 and therefore the molecular formula will be CuSO4 • 5H2O.

Materials: • Hot Plate • Beam Balance • 100mL Beaker
• Scoopula • Stirring Rod • Beaker Tong
• Hydrated Copper (II) Sulfate

Procedure:
1. Prepared a table to record observations.
2. Carefully plugged hot plate into desk outlet and turned on to medium heat.
3. Measured the mass of the beaker and the stirring rod. Recorded the mass in the table.
4. Added 3 g to 5 g of hydrated copper (II) sulfate to the beaker.
5. Measured the mass of the beaker with the hydrated copper (II) sulfate. Recorded the mass in table.
6. Heated the beaker with the hydrated copper (II) sulfate, stirred occasionally, until the crystals lost their blue colour.
7. Removed the beaker with the beaker tongs and allowed it to cool.
8. Found the mass of the beaker with the white powder. Recorded the mass in the table.

Results:

Substance Mass
Empty beaker and glass rod 53.02 g
Beaker, glass rod, and hydrated copper sulfate 57.34 g
Beaker, glass rod, and anhydrous copper sulfate 56.165 g
Hydrated copper sulfate 4.32 g
Anhydrous copper sulfate 3.145 g
Water 1.175 g

Questions:

1. Determine the percent by mass of water in your sample of hydrated copper (II) sulfate.

2. Based on your data, determine the molecular formula of CuSO4 • xH2O.

3. Suppose that you heated a sample of hydrated ionic compound in a test tube. What might you expect to see inside of the test tube, near the top of the test tube? Explain.

If I heated a sample of a hydrated ionic compound in a test tube, inside the test tube I would expect to see water vapour, since the H2O would be evaporating when heated in the hot plate.

4. Suppose that you did not completely convert the hydrate to the anhydrous compound. Explain how this would effect:

a) The calculated percent by mass of water in the compund.

If I didn't completely convert the hydrate to the anhydrous compound, the calculated percent by mass of water int hecompound would be less. This is because the mass of the water would be less due to the mass of the anhydrous copper sulfate would be more. Thus it is not completely anhydrous. Therefore, if I went to subtract the hydrated by the anhydrous copper sulfate to get the mass of the water, it would be less.

b) The molecuar formula you determined.

If I didn't completely convert the hydrate to the anhydrous compound, the molecular formula would also be less since the mass of the water would be less due to the mass of anhydrous copper sulfate being more). Therefore, your molecular formula (and x) would be less.

5. Suggest a cource of error (not alraedy mentioned) that would result in a value of x that is:

a) higher than the actual value.

A course of error that would result in a value of x that is higher than the actual value is if you had not measured the masses correctly. If you had measured the mass to be more than it actually was, than your x would end up being higher than the actual value.

b) lower than the actual value.

A source of error that would result in the value of x that is lower than the actual value is if you had not measured the masses correctly. If you had not measured the compound with the stirring rod inside, than the mass would be less than it actually was. Thus, your x would be lower than the actual value.

6. List 5 specific safety precautions pertinent to this experiment.

Pertinent to this experiment, there were some safety precautions to be taken into consideration. Firstly, your hair would have to be tied back since you were working with a hot plate. Secondly, you had to keep the aisles clear in case of an emergency, so that the teacher can have easy access to you. This also includes keeping your stools and bags clear of the aisles. Thirdly, you had to use a beaker tong to place the beaker ont op of the beam balance, since it was too hot to touch. Fourthly, you had to check your beaker for any cracks, since if it had any, then it would have a great chance of it exploding when heated. Lastly, you had to constantly moniter the hot plate and make sure not to touch it since you would burn yourself if you did.

Conclusion: In conslusion, the hypothesis has been proven to be incorrect since the hypothesis was that the variable x will be 5. However, through calculations, it was proven the correct answer for variable x is 3. Therefore, the molecular formula of the hydrate is CuSO4 • 3H2O.

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