Formula of Compound

[xwrathbringerx]

Hey guys

If all the hydrogen comes from water of crystallisation, how many molecules of water are there in a formula unit of this compound? Hence deduce the formula of the compound in the usual hydrate form (e.g. Na2CO3. 10H2O)

Could someone please explain the question to me? I can't seem to understand what it's asking for or how to do it.

[Sonic]

Hey guys

If all the hydrogen comes from water of crystallisation, how many molecules of water are there in a formula unit of this compound? Hence deduce the formula of the compound in the usual hydrate form (e.g. Na2CO3. 10H2O)

Could someone please explain the question to me? I can't seem to understand what it's asking for or how to do it.

The number of water molecules in a hydrate is given by the number in front of H2O in the question. In your example it is 10.

If by formula unit you mean moles then there are 10 moles of water and since each mole of water contains Avogadro's Number of molecules you will have 10 * 6.022*10^23 = 6.022*10^24 molecules.

If you meant moles of hydrogen then since a water molecule has two atoms of hydrogen to one of water the amount of hydrogen atoms is given by (2/3) * 6.022* 10^24.

Sometimes you'll see a prefix instead of number and then you'd be expected to know the number from the word. Below is a list of common ones:

mono =1
di = 2
penta = 5
hexa = 6


If you need to know the value of x in CuSO4.xH2O then normally you'd find the mass of the hydrate and subtract the mass of CuSO4 then divide by 18 to find x which should be an integer (5 is common in CuSO4)