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Properties such as boiling point, freezing point and vapour pressure of a pure solvent change when solute molecules are added to get homogeneous solution. These are called colligative properties. Applications of colligative properties are very useful in day-today life. One of its examples is the use of ethylene glycol and water mixture as anti-freezing liquid in the radiator of automobiles. A solution M is prepared by mixing ethanol and water. The mole fraction of ethanol in the mixture is 0.1.

Given:

Boiling point elevation constant of water =0.52 K kg/mol Boiling point elevation constant of ethanol = 1.2 k kg/mol

Boiling point of water =373 K Boiling point of ethanol =351.5 K

The boiling point of this solution is?

Now, I am aware that for a solution of a non-volatile solute in a solvent The elevation in boiling point is given as

$\Delta\mathrm{T}=im\mathrm{K}_{b}.$

In this case,which is the solute and which the solvent?

The material that I am studying from considers ethanol to be the solute and water,the solvent.And thus,calculates the elevation in boiling point of water. Therefore,the new boiling point is higher than that of pure water

My problem with this is,since the boiling point of a mixture of ethanol and water,for all compositions,lies between that of water(maximum) and ethanol(minimum), how can a mixture of water and ethanol of 0.1 molarity (by ethanol) have a higher boiling point than that of water?

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  • $\begingroup$ I heard someone say "non-volatile". That's the key. $\endgroup$ – Ivan Neretin Dec 14 '16 at 14:56
  • $\begingroup$ @IvanNeretin Indeed,the formula I stated appeared with regards to elevation of boiling point when a non-volatile solute is added to a solvent. With regards to a binary mixture of volatile liquids,I did not encounter any such formula .Only ones relating to changes in vapour pressure( Raoult's law). But , we know the boiling point does change for a liquid-liquid mixture (from b.p of lower boiling liquid to b.p of higher boiling liquid). Is there then a law/formula which relates this change with composition? $\endgroup$ – Cyka Dec 14 '16 at 16:10
  • $\begingroup$ Your best bet is to use the approximate expressions for vapor pressures of both compounds, equate their sum to atmospheric pressure, and solve for temperature. $\endgroup$ – Ivan Neretin Dec 14 '16 at 18:10
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Are you sure that is the right equation to answer the question? It only holds true for non-volatile compounds. You should be able to use Raoult's Law. Perhaps this helps http://www.chemguide.co.uk/physical/phaseeqia/idealpd.html

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