[tiennhan]

#30 Compared to the freezing point of 1.0 M KCl(aq) at standard pressure, the freezing point of 1.0 M CaCl2(aq) at standard pressure is

(1) lower (2) higher (3) the same

The answer is (1). the explanation should be simply enough, I just cann't find the relation to what they're asking.

Thanks in advance!

[Yggdrasil]

What is the concentration of solute ions in 1.0M KCl and what is the concentration of solute ions in 1.0M CaCl₂?  Hint: they are not the same.

[tiennhan]

i would assume that the molarity is the same so therefore the concentration is the same...but that's wrong.

Is it the fact that when KCI ionizes u get K and Cl but with the other one you get 2 Cl-s?

But then i still don't see the connection to the freezing pt.

[Yggdrasil]

Is it the fact that when KCI ionizes u get K and Cl but with the other one you get 2 Cl-s?

Yes!  See the following equations:

KCl(s) --> K⁺(aq) + Cl^-(aq)
CaCl₂(s) --> Ca²⁺(aq) + 2Cl^-(aq)

When a mole of KCl (not KCI) disolves in water you get two moles of solute molecules: one mole of K⁺ and one mole of Cl^-.  However, when one mole of CaCl₂ disolves in water, you get three moles of solute molecules:  one mole of Ca²⁺ and two moles of Cl^-.

But then i still don't see the connection to the freezing pt.

Please review colligative properties (especially freezing point depression) in your chemistry text book.  Alternatively, see http://en.wikipedia.org/wiki/Colligative_properties

[tiennhan]

if so, then can I assume as a general rule that if there's more ionized particles in the solute, than it decreases the freezing pt. Was that what the wikipedia was trying to say?

[Yggdrasil]

Basically.  If you want a more detalied explanation of why this drop in freezing temperature occurs, please see my post in the thread here: http://www.chemicalforums.com/index.php?topic=9380.0