[mp420]

hi i'm studying the mole and i've got a question for homework under the heading percentage mass and empirical formulae.

5) Samples of the following hydrates were weighed, heated to drive off the water of crystallisation, cooled and reweighed. From the results calculate the value of a,b.c;
    a. 0.869g of CuSO4.aH2O gave a residue of 0.556g
    b. 1.173g of CoCl2bH2O gave a residue of 0.641g
    c. 0.904g of Pb (C2H3O2)2.cH2O gave a residue of 0.774g

I know how to calucalte empirical forumlae and percentage mass but i don't understand what this particular question is asking for. Am i supposed to calcucate the % mass, empirical formula, number of moles etc.. what?
 
I would really appreciate some help because i'm confused as to what i'm supposed to work out!

[xiankai]

a. the mass loss is entirely water, isnt it? convert the mass of CuSO4 and H2O into moles, so u can see the molar fraction. ( the ratio of CuSO4 to H2O )

b. same method as above.

c. etc.

[Karakth]

The question is asking for the number of moles of water of crystallisation in each case.

[xiankai]

u  have to compare it to the number of moles of substance... to get the empirical formula?

[Karakth]

I do not see any reference in the question to empirical formulae.

Do not confuse the question with the chapter it is in.

[Borek]

Question wording is misleading. Perhaps not wording, but the symbols used. Try this:

Samples of the following hydrates were weighed, heated to drive off the water of crystallisation, cooled and reweighed. From the results calculate the value of x, y, z:
    a. 0.869g of CuSO4.xH2O gave a residue of 0.556g
    b. 1.173g of CoCl2.yH2O gave a residue of 0.641g
    c. 0.904g of Pb (C2H3O2)2.zH2O gave a residue of 0.774g

[mp420]

thanks, that makes soo much more sense!