I keep getting errors in a simple copper sulfate experiment

[DDNow]

Homework Statement

I need to find the total number of water molecules per copper sulfate molecule. The answer is 5. But, I keep getting ~4 with the average of two dehydrating experiments.

2. Homework Equations with my Attempts

I am just using simple factor equations for these. I am converting from the weight of water that I liberated to moles. Then, comparing moles to find experimental values for the number of water molecules in the copper sulfate. I hope you understand. This is the first time I've tried to use LateX

Test 1

0.340g H₂O * \frac{1 mol H_{2}O}{18.016g H_{2}O} = 0.0189 mol H₂O

0.6449g CuSO₄ * \frac{1 mol CuSO_{4}}{159.62g CuSO_{4}} = 0.00404 mol CuSO₄

Experimental number of H₂O Molecules is 4.68 or \frac{0.0189 mol H_{2}O}{0.00404}

That's fine for that one, I can round up to 5 for the ratio.

For the second test, I used the same equations with the measured weight of 0.328g H₂O and 0.729g CuSO₄.

This gives 0.0182 mol H₂O and 0.00457 mol CuSO₄.

The experimental value for the number of H₂O molecules comes out to 3.98.

This is where I am not doing so well (obviously). When I average these two and round off, I get 4. To the best of my knowledge, copper sulfate tetrahydrate doesn't exist.

So, am I just dealing with imperfect lab situations (technique, materials, etc..), or is there a fundamental flaw in my approach. Thanks for your time.

Mike

 

[GCT]

It's the former.