Hydrates

[TheShapeOfTime]

If I have the mass of a hydrate before and after the hydrate was evaporated away, how can I find the ratio of molecules?

For example: CoCl2 * xH2O (The * is meant to be a dot). I need to find "x".

[chem_tr]

Yes, the hydrate formulae are found in this way, .by successive evaporations and subsequent weight measurements until a stable reading is achieved.

Suppose that you started with a grams of CoCl_2\cdot xH_2O and after several steps, you get b grams of CoCl_2. Then it means that b-a grams of water was evaporated in the process, just divide it by 18 (molar mass of water) to find the overall x. Note that you may not be very precise in this calculation, so if you find x=2,5, you may conclude that it can be regarded as x=3, etc.

[TheShapeOfTime]

Are you sure it's not "a - b"? If I try it as you said then I get -0.041 (a = 1.62, b = 0.88). This doesn't seem correct. Am I doing something wrong?

[Mertas]

Yes, what chem_tr means that if a is total weight before evaporation, then b is to be the weight after evaporation... This difference gives the amount of water. (Definitely positive)

Of course, we suppose the hydrate compound is pure...

[TheShapeOfTime]

Yes, what chem_tr means that if a is total weight before evaporation, then b is to be the weight after evaporation... This difference gives the amount of water. (Definitely positive)

Of course, we suppose the hydrate compound is pure...

CoCl_2 \cdot xH_2O

1.62 is the mass before evaportation
0.88 is the mass after evaportation

1.62 - 0.88 = 0.74
0.74 / 18.02 = 0.041

How can this be right?

[chem_tr]

Oops, I should have written a-b of course. The positive difference between these two measurements gives the amount of water evaporated.

About the difference, 0.041 moles of water is present in this compound, namely CoCl_2 \cdot xH_2O. The molar amount of the initial compound is not known, but we may consider that 0,88 grams of CoCl_2 is present, you can find the molar mass from Co:58.93 and Cl:35.45 grams/mol. You then set up a proportion equation to find how many moles of water are present in one mole of CoCl_2. This will give \displaystyle x you're looking for.

I recommend that you use greater amounts of salt and multiple determinations to minimize errors. For example, do the analysis triplicate at one time and use at least 5 or 10 grams of sample, then average the findings you obtained. This will be better.

[TheShapeOfTime]

Oops, I should have written a-b of course. The positive difference between these two measurements gives the amount of water evaporated.

About the difference, 0.041 moles of water is present in this compound, namely CoCl_2 \cdot xH_2O. The molar amount of the initial compound is not known, but we may consider that 0,88 grams of CoCl_2 is present, you can find the molar mass from Co:58.93 and Cl:35.45 grams/mol. You then set up a proportion equation to find how many moles of water are present in one mole of CoCl_2. This will give \displaystyle x you're looking for.

I recommend that you use greater amounts of salt and multiple determinations to minimize errors. For example, do the analysis triplicate at one time and use at least 5 or 10 grams of sample, then average the findings you obtained. This will be better.

Could you tell me a bit more about this proportion equation?

[chem_tr]

Okay, first find how many moles are there in 0.88 grams of \displaystyle CoCl_2. Then calculate this: "If there are n moles in 0.88 grams, 1 mole would be X". Then set up a second calculation, "if 0.041 moles of water is present in n moles of compound, how many moles of water are present in 1 mole of compound?"

I think you'll be able to do these.

[TheShapeOfTime]

First calculation:


CoCl_2 = 129.83 g/mol



\frac{0.88}{129.83} = 0.0068 mol


Second Calculation:


\frac{0.041}{0.0068} = 6.0


I'm not sure what you wanted for `X', and I don't think I did the second calculation right.

[chem_tr]

Congrats, and you're right about X, we don't need to use this, as we know that \displaystyle CoCl_2 is 129.83 g/mol. I did the same calculation and found 6.0, and it is very characteristic for cobalt to coordinate six water molecules in the form \displaystyle [Co(H_2O)_6]Cl_2.

[TheShapeOfTime]

So from all this we get that the formula must be CoCl_2 \cdot 6H_2O? Thanks for all your help!