[methic]

Hey everybody. Again.

So, I was trying to find the number of waters of hydration in a hydrate. And I got it wrong. I came up with 1.25, and I know that doesn't make sense. I don't know if I set up the question wrong or what, but the answer is supposed to be 2. Just wondering if someone could tell me how that works out.

A Hydrate of Copper(II) chloride has the formula CuCl₂·xH₂0. The water in a 3.14g sample was driven off by heating. The remaining sample had a mass of 2.69g. Find the number of waters of hydration (x)

So, I got the total mass of water by subtracting 2.69g from 3.14g, which gave me 0.45g H₂O. I got 0.025 mol of water from that.

I then found the moles of CuCl₂ in 2.69g (the amount after evaporation), which was 0.02 mol.

Then I divided the moles of water by the moles of CuCl₂, and got 1.25. So, I read that as 1.25 water molecules per one unit of CuCl₂. I understand you can't have .25 of a water molecule, so... eh? Just wondering how to do these types of problems. Where I messed up, etc. Any help would be appreciated.

[Borek]

There is no rule that states you must have integer number of water molecules per molecule of other substance. It is quite common that they are just in the ratio given by some small integer numbers. In this particular case that would be something like

4CuCl₂·5H₂O

IUPAC has a specific naming rule for such situations, this compound should be named

copper(II) chloride - water (4/5)

(compare http://www.iupac.org/publications/books/rbook/Red_Book_2005.pdf, page 92/3).

[methic]

Thanks for replying.

So, did I do the question correctly? Based on the numbers, I don't understand how one gets 2H₂O per CuCl₂.

[methic]

Thanks for replying.

So, did I do the question correctly? Based on the numbers, I don't understand how one gets 2H₂ per CuCl₂.

Oh hey again! Turns out I'm dyslexic. The initial mass was 3.41g; not 3.14g. I wrote the freaking question down wrong. I spent hours trying to figure out what I did wrong!

 

Apologies.

Appreciate the previous reply, though. It gave me some good info.

[Borek]

LOL, your result was so nicely close to 4:5 I was sure your result was the one expected