# Exercise about an aqueous solution (containing a halogen)

• Chemistry
• Ako9

#### Ako9

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.)

• Ako9
Last time I checked rubidium wasn't a halogen The molality can be found exactly using the information given and the freezing point of water. You didn't use this number?

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.

• Ako9
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.

• Ako9
% is OK as long as used correctly, it is just misused here.