GC Achiever question

[299678]

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Q. Determine the value of X if 10g of CUSO4XH2O have been dehydrated to 6.4 g unhydrous CUSO4?

What I did was calculate the gram of dehydrated water (10-6.4 = 3.6g), so 3.6 g of water give us 0.2 mole, so x = 0.2

However, the solution on achiever divided it by the moles of CuSO4, so 0.2/0.04 = 5 so X = 5

Someone can explain this to me? Why we should divide it by number of moles of CuSO4?

 

[Cool Beans]

Hey kpark,

The question should have read what is the number (not volume) 'X' in the hydrate of copper (II) sulfate (CuSO4.XH2O)?

As you showed there are 5 moles of water for every 1 mole of CuSO4 (why they divided it (.2/.04)) so the correct formula of the hydrate is CuSO4.5H2O.

A little miswording in the question. Hope this helps.

 

[299678]

Thank you Cool Beans for your input, but I still don't get it
My main question is why we should divide it by 0.04
any other thought?

 

[UndergradGuy7]

6.4 grams cuso4 / 159.61 g/mol cuso4 = 0.04 moles of cuso4

So we have the moles of h2o and cuso4. We want the ratio of h2o to 1 cuso4.

So moles of h2o/moles of cuso4.

 

[299678]

+1 Thank you undergradguy7