# Chemistry Purity Problems with solutions help

I have a test tomorrow and I thought I knew how to do these types of problems but my answers are all wrong.

Here is the question:
The solubility of CaSO4 at 25*C is 1.61x10^-2 mol/L. What mass will dissolve in 50.0 mL of water if the sample is only 20.0% pure?

Heres what I did. First I found moles of CaSO4:
n=CV
n=1.61x10^-2 x 0.05L
n=.00805 mol

Then I converted moles of CaSO4 to mass (grams):
m=0.00805mol x 136g/mol
m=1.0948g

Then I did a conversion of 100% pure to 20%:
1.0948g/100 = x/20
(Cross multiply)
100x = 21.896
x = .219g
Which means the sample of CaSO4 that is 20% pure should have a mass of .219g but according to the answers that is incorrect. Can someone tell me what I am doing wrong and how I can fix it. Thanks

Last edited:

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I like Serena
Homework Helper
Welcome to PF, loz123!

I interpret your problem statement as that you need to find a total mass of which 20% is actually CaSO4.
As it is, 20% of .219 g is not 1.0948 g.

Borek
Mentor
I find this question ambiguous - or intentionally tricky. You will dissolve 1.09g of CaSO4p/sub] in 50 mL of water regardless of the sample purity (unless sample contains other sulfates or other soluble calcium salt - but that's another problem).