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Q & A: freezing point of water with some salt

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Most recent answer: 02/26/2011
what is the freezing point of saltwater if there is 1/2 cup of water and 1/4 tsp of salt?
- Samantha (age 48)
That 1/4 tsp is about 1.5 gm. That's about 0.038 moles of NaCl. The 1/2 cup is 118 ml of water. So you have about 0.32 molar NaCl. That's 0.32 molar each of Na+ and Cl-. For low concentrations, the freezing point is reduced 1.8C for each molar solute. So that would leave about -0.6C for the freezing point. ent of NaCl in water vs. concentration, I guess the reduction isn't quite that big because the ion concentration isn't quite low enough for that approximation to be accurate. It looks like the activity coefficient is only about 0.75 or so, so the freezing point should be around -0.45C.

You might check some of our other answers for a discussion of the physical reasons behind this effect.

Mike W.

(published on 02/26/2011)

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