What mass of calcium chloride must be added to 100 grams of water to make it fre

Chemistry
Tutor: None Selected Time limit: 1 Day

What mass of calcium chloride must be added to 100 grams of water to make it freeze at 269 K?

please help with detail

May 19th, 2015

Hello!  Hope this helps:

The equation you will use is Blagden's Law:

deltaTf=Kf*b*i

deltaTf=freezing point depression=Tf(water)-Tf(solution)

Tf(water)=273K, Tf(solution)=269K

Kf=cryoscopic constant = 1.853K*kg/mol for water

b=mol CaCl2/kg of water=mol CaCl2/.1kg water

i=number of ion particles per individual molecule of solute(CaCl2) = 3

deltaTf=Kf*b*i

273-269=(1.853)*(mol CaCl2 / .1)*3

4=55.59*mol CaCl2

mol CaCl2 = .072 mol

Now use the molar mass of CaCl2, which units are in grams per mole, then multiply by moles to get answer in grams of CaCl2.

CaCl2: 110.98g/mol * .072mol = 8grams (approximately)

Let me know if you have any questions.  Please give me a rating so I know how I'm doing.  Best of luck to you :).

May 19th, 2015

What calculations gave us .072 mol?

May 19th, 2015

4 / 55.59

May 19th, 2015

ur good thanks!

May 19th, 2015

You're welcome!  Give me a good rating if you get the chance :).  Good luck with everything else!!!

May 19th, 2015

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