Mass of a hydrate before and after the hydrate was evaporated away

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In summary, to find the ratio of molecules in a hydrate, you can use successive evaporations and weight measurements. Take the difference between the initial and final mass and divide it by the molar mass of water to find the overall x. To minimize errors, it is recommended to use larger amounts of salt and multiple determinations. To find the proportion equation, first calculate the number of moles in the initial mass, then use that number to find the number of moles of water in one mole of the compound. The resulting ratio can be used to determine the formula of the hydrate.
  • #1
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".
 
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  • #2
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 [itex]a[/itex] grams of [itex]CoCl_2\cdot xH_2O[/itex] and after several steps, you get [itex]b[/itex] grams of [itex]CoCl_2[/itex]. Then it means that [itex]b-a[/itex] 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 [itex]x=2,5[/itex], you may conclude that it can be regarded as [itex]x=3[/itex], etc.
 
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  • #3
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?
 
  • #4
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...
 
  • #5
Mertas said:
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...

[tex]CoCl_2 \cdot xH_2O[/tex]

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

[tex]1.62 - 0.88 = 0.74[/tex]
[tex]0.74 / 18.02 = 0.041[/tex]

How can this be right?
 
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  • #6
Oops, I should have written [itex]a-b[/itex] of course. The positive difference between these two measurements gives the amount of water evaporated.

About the difference, [itex]0.041[/itex] moles of water is present in this compound, namely [itex]CoCl_2 \cdot xH_2O[/itex]. The molar amount of the initial compound is not known, but we may consider that [itex]0,88[/itex] grams of [itex]CoCl_2[/itex] is present, you can find the molar mass from [itex]Co:58.93[/itex] and [itex]Cl:35.45[/itex] grams/mol. You then set up a proportion equation to find how many moles of water are present in one mole of [itex]CoCl_2[/itex]. This will give [itex]\displaystyle x[/itex] 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.
 
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  • #7
chem_tr said:
Oops, I should have written [itex]a-b[/itex] of course. The positive difference between these two measurements gives the amount of water evaporated.

About the difference, [itex]0.041[/itex] moles of water is present in this compound, namely [itex]CoCl_2 \cdot xH_2O[/itex]. The molar amount of the initial compound is not known, but we may consider that [itex]0,88[/itex] grams of [itex]CoCl_2[/itex] is present, you can find the molar mass from [itex]Co:58.93[/itex] and [itex]Cl:35.45[/itex] grams/mol. You then set up a proportion equation to find how many moles of water are present in one mole of [itex]CoCl_2[/itex]. This will give [itex]\displaystyle x[/itex] 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?
 
  • #8
Okay, first find how many moles are there in 0.88 grams of [itex]\displaystyle CoCl_2[/itex]. 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.
 
  • #9
First calculation:

[tex]
CoCl_2 = 129.83 g/mol
[/tex]

[tex]
\frac{0.88}{129.83} = 0.0068 mol
[/tex]

Second Calculation:

[tex]
\frac{0.041}{0.0068} = 6.0
[/tex]

I'm not sure what you wanted for `X', and I don't think I did the second calculation right.
 
  • #10
Congrats, and you're right about X, we don't need to use this, as we know that [itex]\displaystyle CoCl_2[/itex] 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 [itex]\displaystyle [Co(H_2O)_6]Cl_2[/itex].
 
  • #11
So from all this we get that the formula must be [itex]CoCl_2 \cdot 6H_2O[/itex]? Thanks for all your help!
 

1. What is a hydrate?

A hydrate is a compound that contains water molecules within its crystal structure. These water molecules are typically bound to the compound through weak chemical bonds.

2. How does a hydrate lose its water molecules?

A hydrate loses its water molecules through a process called evaporation. This occurs when the hydrate is heated, causing the water molecules to become gaseous and escape from the compound.

3. What is the mass of a hydrate before evaporation?

The mass of a hydrate before evaporation is the total mass of the compound, including the mass of the water molecules bound to it. This can be determined by measuring the mass of the compound before heating.

4. What is the mass of a hydrate after evaporation?

The mass of a hydrate after evaporation is the mass of the compound remaining after all the water molecules have evaporated. This can be determined by measuring the mass of the compound after heating and subtracting the mass of any remaining water molecules.

5. How is the mass of a hydrate affected by evaporation?

The mass of a hydrate is affected by evaporation as the water molecules bound to the compound are lost, resulting in a decrease in overall mass. The amount of mass lost will depend on the number of water molecules present in the original compound.

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