Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: cem1982 on February 09, 2008, 03:30:04 PM
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I am pretty new to chemistry and have been trying to teach myself. I ran across a problem that I need a little help on. I have basically done all of the problem except the last part. The problem states:
The student is given a sample of a blue copper sulfate hydrate. She weighs the sample in a dry covered crucible and obtains a mass of 21.587g for the crucible, cover, and sample. The mass of the empty crucible and cover is 20.623g. Heating the crucible to drive off the water of hydration. On cooling, she finds the mass of crucible, cover and contents to be 21.240g. The sample was converted in the process to very light blue anhydrous CuSO4
Questions and my answers were:
A. What is the mass of the hydrate sample? . 964g hydrate
B. What is the mass of the anhydroues CuSO4? .617g CuSO4
C.What is the mass of water driven off? .347g H20
D. What is the percent of water in the hydrate? 35.9%
This is where I ran into trouble
E. How many grams of water would there be in 100.0g of hydrate? How many moles?
I used 35.9g of H20 and im trying to get to moles.
Does this look about right?
100.0g Sample(6.022X10^23/35.9 g) = 1.677X 10^24 mol
I'm not sure exactly how I would go about setting that up. Any help would be greatly appreciated. Thanks in advance.
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To find the number of moles in a substance, you divide the mass of the substance by that substance's molar mass.
So, you have 35.9g of water and water has a molar mass of 18.0g/mol.
35.9g * (mol/18.0g) = 1.99 mol
notice that in the calculation grams appear in the numerator and denominator so they cancel out. This is how you can remember to divide by molar mass instead of multiply by molar mass.
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Thank you so much!