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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

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

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