Determining relative freezing points

Discussion in 'MCAT Study Question Q&A' started by Lindsey Freeman, 05.15.14.

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1. Lindsey Freeman

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The question gives you the following salts in 100 mL of deionized water: Silver nitrate, Lithium chloride, sodium Iodide, and Potassium carbonate. It asks if all solutions are cooled at an equal rate, which solution would remain unfrozen for the longest time?

The answer is Lithium chloride solution, but I don't understand why?

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3. CzarcasmHakuna matata, no worries. 2+ Year Member

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Strange. Are you sure they asked for the longest time, not shortest? You didn't specify whether moles or grams of salts were given. Big difference there because NaI has the largest molecular weight, so if all salts weighed the same amount, it would have the smallest number of moles. And because the first 3 salts have a vant hoff factor of 2, NaI would have the smallest change (freezing point depression) and therefore freeze the soonest relative to some higher temperature than the other solutions.

EDIT: Not strange at all. For some reason I thought you said the answer was NaI, but after re-reading it a second time (seeing DrknoSDN's post), I realize it makes absolute sense. LiCl, would have the lowest molecular weight and therefore for the same given weight, the greatest number of moles and the largest effect on freezing point (depression). In this case, since it would decrease the most, it would last the longest as a liquid relative to some higher temperature (as we cool down). But all this is provided they gave weights in grams, not equal moles.

Last edited: 05.15.14
4. DrknoSDN

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After thinking about this one for a while I'm guessing Czarcasm is right about giving mass instead of moles. It could have given say (10g in 100mL of DI water), then because Lithium chloride is by far the lowest MW (@ 42g/mol vs 138, 150, and 170), even with the van't hoff factor it would still have the greatest number of moles and cause the greatest freezing point depression, freezing the slowest.

Only things that alter freezing point depression are the cryoscopic constant (all the same for the DI water), molality, and van't hoff factor.

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5. Lindsey Freeman

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Got it, thank you!