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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
The freezing point of isopropyl is -127F (?). Water is 32F. If one adds a solute to water each molal reduces water's freezing point by 1.86C. So, 40% isopropyl:
400g/60g/mole = 6.67 mole. 0.6kg of solvent (water) means we have a 11.11 molal solution
1.86 x 11.11 = -20.7C
However, if we have a 90% solution one would get 150 molal * 1.86 = -279C, which is below the freezing point of the alcohol. This is not correct.
So, the question is when does the effect of the solute depressing the freezing point start to taper off? I also can't see the 90% alcohol having a freezing point equal to pure isopropyl. Also, wouldn't the water just freeze out of the solution?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
There are Rules based on fundamental physics, anywhere, anytime, and there are Rules that give a near enough result over a limited range of conditions, I guess that the 1.8C/mole is of the second type, most 'Rules' are. 'solute' probably means 'high melting point non-reactive solid'
The mixture I'm familiar with is Lead/Tin, Lead melts at 327C, Tin melts at 232C but 63/37 Pb/Sn melts at 183C.
its the two metals form an alloy, alcohol and water form a sort of larger molecule so sort of similar ?
Based on my in-depth knowledge of this physical Universe....... I predict that compared to 100% Water and 100% alcohol a graph of properties vs. concentration, mixtures will form a straight or curved line dipping and/or peaking at completely unexpected places for completely unexpected reasons.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
This might help but essentially there's a logical problem with the way you got to the answer. For dilute solutions each mole of solute depresses the fp of water by 1.86C However there's an equivalent, but different value for the freezing point depression of IPA.
Which one would be right for a mixture of the two liquids?
The answer is impractical or impossible to calculate so the values are measured.
For ethanol and water the eutectic is about -125C I guess the result would be similar for IPA/water.
Some solvents "unmix" when cooled and you get two liquid layers.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Thanks for the replies.
Actually, solute refers to the smaller substance dissolved in a larger one. For example, oxygen is the solute dissolved in nitrogen to form the mixture we call air. I relearned this when I was helping my daughter study this subject in science. It does not have to be a solid.
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calculating freezing point of a mixed solution
