The heat of vaporization of pure methanol (CH3OH) is 35.2 kJ/mol at its boiling
ID: 994803 • Letter: T
Question
The heat of vaporization of pure methanol (CH3OH) is 35.2 kJ/mol at its boiling point of 64.6 oC (when pressure is 1 atm). Suppose you add 183 grams of a fully soluble, non-dissociating substance to 1000 g of methanol. You measure the boiling point temperature of the solution to be 0.192 oC higher than the pure solvent boiling point temperature. What is the molar mass of the substance that you put into solution? You may assume that Hvap of methanol is constant over this temperature range. The molar mass of methanol is 32.05 g/mol.
Explanation / Answer
DTb = i*Kb*m
kb of methanol = 0.83 K/(mol/L)
0.192 = 1*0.83*(183/x)*(1000/1000)
molarmass of substance = x = 791.09 g/mol
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