Exercise about an aqueous solution (containing a halogen)

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Discussion Overview

The discussion revolves around the calculation of molality in an aqueous solution containing a halogen. Participants are addressing potential errors in the calculations related to the mass of solute and solvent, as well as the implications of using percentage concentrations in the context of freezing point depression.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant outlines their approach to calculating molality, expressing uncertainty about the results and questioning the identification of the solute as a halogen.
  • Another participant agrees with the calculation but raises concerns about the interpretation of "x% solution" and its implications for accuracy.
  • A third participant humorously points out that rubidium is not a halogen, suggesting a misunderstanding in the identification of the solute.
  • Some participants emphasize the importance of using the freezing point of water in the calculations, suggesting it was not utilized correctly.
  • There are repeated assertions that the percentage concentration used in the calculations may be unreliable, leading to confusion and incorrect results.
  • One participant expresses frustration with the use of percentage concentrations, describing them as problematic in this context.
  • Another participant defends the use of percentage concentration, stating it can be acceptable if applied correctly.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the reliability of the percentage concentration used in the calculations. There are competing views on the correctness of the calculations and the identification of the solute.

Contextual Notes

There are unresolved assumptions regarding the definition of the solution's concentration and its impact on the calculations. The discussion highlights potential discrepancies in the interpretation of mass ratios and the implications for identifying the solute.

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Misplaced Homework Thread
Homework Statement
"An aqueous solution, with 5% of CaX_2 (X is an halogen), freezes at t=-1.396°C. K_cr=1.860. Find X."
Relevant Equations
$$\Delta t = K_{cr}\cdot m \cdot i$$
I can't find my mistake, can you help me?

I suppose that in 1000g of solution there are 50g of solute and 950g of solvent.
Then I find the molality
$$m= \frac{50}{MM} \cdot \frac{1}{0.950}$$
So I can find the Molar Mass of the compound using the relation $$\Delta t=K_{cr}\cdot m \cdot i$$ where i is the van 't Hoff coefficient which is 3 in this case.
At the end I don't find an halogen (Rubidium is the closest) so I assume something is wrong but I can't see what.
 
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I think you've done the calculation correctly - at least, I agree with your answer. You get a halogen if you assume there are 50g solute to 1000g water. I'm never quite sure what people mean when they say "x% solution" with no further specification. Perhaps they think that this solution is sufficiently dilute that you can make this approximation, but that seems inconsistent with measuring T to 4 sig figs. (By the way, we usually use T for temperature, t for time.)
 
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Last time I checked rubidium wasn't a halogen :wink:
 
The molality can be found exactly using the information given and the freezing point of water. You didn't use this number?
 
Mayhem said:
The molality can be found exactly using the information given and the freezing point of water. You didn't use this number?
They did, but it doesn't produce a reasonable answer. Turns out the 5% part is unreliable, try to do the calculations assuming 50/950 and 50/1000 and you will see where the problem is.
 
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Borek said:
They did, but it doesn't produce a reasonable answer. Turns out the 5% part is unreliable, try to do the calculations assuming 50/950 and 50/1000 and you will see where the problem is.
I did the calculation and seem to be getting nonsense numbers. Concentration in % is honestly the devil.
 
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% is OK as long as used correctly, it is just misused here.
 

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