[noiseordinance]

Hi there. My instructor did not show us how to do a certain problem and expects us to just know, according to our homework. Can someone help me out with what I would need to do to answer the question in a manner that I'd be able to reproduce on a test?

"You want to determine the value of x in hydrated iron (II) sulfate, FeSO4 x xH20. In the laboratory you weigh out 1.983g of the hydrated salt. After thorough heating, 1.084g of the anhydrous salt remains. What is the value of x?"

I have no clue what to do.... I'm guessing maybe the empirical formula comes into play, or maybe the mass percent of water?

Thanks for any help.

[macman104]

Close, you have a compound FeSO₄ * xH₂O.

When you heat it, you have driven off all of the water, so the weight loss is all going to be water.  From there you can also find how many moles of water you have, and also the moles of FeSO₄ you had (since you have the mass of water lost).  Then use ratios to figure out how many moles of H₂O there are to 1 mole of FeSO₄

[noiseordinance]

So let me see if I get this. I take the original weight of the compounds, 1.983g - 1.084g of the remainder, giving me 0.899g of H₂O. Then I figure out the molar mass of H₂O, about 32g, and figure out how many moles are in 32g? 0.899 (1 mol / 32g) = 0.028mol. Am I on the right track?

[macman104]

Yup, and you can also find out how much FeSO₄ is there, and then you can use molar ratios.  For example, if you had 0.2 moles of FeSO₄ and 0.8 moles of H₂O.  You would see that there are 4 parts of water to every one part FeSO₄, so the formula would be FeSO₄ * 4 H₂O.  See if you can adapt that to your own results.