[Rutherford]

CaCl₂ can crystallize as a dihydrate. Above the equilibrium mixture (t=20°C):
CaCl₂---------CaCl₂*2H₂O  p(H₂O)=0.00045 bar.
The pressure of water on t=20°C is 2338 Pa. How many percents of the moisture, present in the dessicator, can combine with CaCl₂?

The bar to Pa is 45 Pa. When I divide 2338 by 2338+45 and the multiply it by 100 I get 98%, the correct answer, but I don't understand the theory behind this, why I had to do in the calculation the thing I done? Any help appreciated!

[XGen]

I'm not really sure what you mean by "The pressure of water on t=20°C is 2338 Pa", but it could be that vapor pressure-wise or equilibrium-wise that partial pressure of water is required, sort of like an equilibrium concentration.

[Rutherford]

It is the vapor pressure. How to solve this as an usual equilibrium problem?

[curiouscat]

Interesting situation.

[tex]
\frac{2338}{2338+45} \cdot 100=98.11\% \\
[/tex]

[tex]
\frac{2338-45}{2338} \cdot 100=98.08\% \\
[/tex]

Are you sure your answer is the right one or just serendipitously close to the right one? 

[Rutherford]

Hehe  ! The answer in the book is just 98%. I got it by the method I wrote and I thought that it is okay, but I didn't understand it. Saw now that it is wrong. I think that I understand the 2nd formula you wrote. There was a pressure of 2338 Pa, and after the equilibrium was established, it was 45 Pa, so the difference reacted.

[curiouscat]

Hehe  ! The answer in the book is just 98%. I got it by the method I wrote and I thought that it is okay, but I didn't understand it. Saw now that it is wrong. I think that I understand the 2nd formula you wrote. There was a pressure of 2338 Pa, and after the equilibrium was established, it was 45 Pa, so the difference reacted.

Yep! That's indeed what I had in mind. No fun giving you the answer directly. 

[sjb]

Interesting situation. Are you sure your answer is the right one or just serendipitously close to the right one? 

Not especially, just recall that for small x; x² is even smaller, so 1+x ≈ 1/(1-x)

[curiouscat]

Interesting situation. Are you sure your answer is the right one or just serendipitously close to the right one? 

Not especially, just recall that for small x; x² is even smaller, so 1+x ≈ 1/(1-x)

Not mathematically interesting.

 Interesting because there was a perfectly reasonable wrong way to get the right answer.