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I recently bought in Italy a bottle of "Alcool puro" which is in fact 95% Alcohol and 5 % Water. So alcohol has a freezing point of -114°C and water a freezing point of 0°C. How can I calculate then the freezing point of the "Alcool puro"?

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In general it is extremely hard to get the freezing point of a mixture purely by calculation. The reason is that the specific molecular interactions of the two substances you have mixed cause pretty much every mixture to need a unique formula for the calculation.

That said, there are some ways to get a rough estimate but these estimates mainly work for (linear) hydrocarbons because those are (fairly) simple to generalize. (google for "freezing point hydrocarbon mixture")

In your specific case, because it is ethanol-water your best way to find out the freezing point is to look up the experimental results of someone else. The engineering toolbox has a dataset on ethanol-water which you could use to get a reasonable estimate for your case (they don't list 95v%, but they do have the whole range from 0-100v% in steps of 10v%).

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  • $\begingroup$ Too bad the ethanol-water eutectic point is just between 90v% and and pure ethanol. $\endgroup$ – Ivan Neretin Jan 17 '16 at 19:48

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