Mathematical way to find number of water molecules in hydrates?

[umer007]

Is there a mathematical way/formula to find the number of water molecules in hydrates? If someone knows the formula, could you plz also show steps to coming to that formula.

 

Thx alot

[budullewraagh]

well...if you take for example, a sample of Al(NO3)3*9H2O

there will be 9 water molecules per aluminum nitrate molecule

[umer007]

no I meant like say you were given the name of the hydrate or like its mass, the anhydrous salts mass, mass of water (all grams) and percentage composition of water in the hydrate. Or molar mass of the hydrate, molar mass of the anhydrate, mass of water in the hydrate (g) and percent composition of water in the hydrate. Then how would you find out the number of water molecules?

 

Actually Im not exactly certain about it either. I was told there is a mathematic formula to determine the number of water molecules in a hydrate. For example epson salt is magnesium sulphate MgSO4. And its hydrated formula is MgSO4*7H2O, but say you werent given the number 7 only MgSO4*nH2O, where n is a whole number. How would you find out what n is, if you were given molar mass of the hydrate and the anhydrous salt. Or what information would you need to find n?

[Tetrahedrite]

How would you find out what n is, if you were given molar mass of the hydrate and the anhydrous salt. Or what information would you need to find n?

It is quite simple, you take the molar mass of the hydrated salt, subtract the anhydrous molar mass, and divide by 18.016 (the molar mass of water)

eg Molar mass MgSO4.nH2O = 246.488g/mol

Molar mass MgSO4 = 120.376g/mol

 

246.488-120.376 = 126.112

 

now divide by 18.016 = 7 waters

[budullewraagh]

take the molar mass of the hydrated salt

if you don't have that you can't find the answer

[Gilded]

"if you don't have that you can't find the answer"

 

That was quite obvious, now wasn't it. ) Of course, if you knew the individual atom masses and... Hmm...

[budullewraagh]

well, im just saying. you can't know the molar mass if you don't know the formula. thus, if you're just given any sample of a hydrate and you don't know the potential hydrates, you cannot quantitatively solve this problem

[Gilded]

Well if you're given the formula of the hydrated compound, but not the molar mass of the compound, you could calculate the approximation of the molar mass if you knew the masses of the individual atoms in it.