Raoult\'s law can be used to determine the molar mass of an unknown substance ba
ID: 1077133 • Letter: R
Question
Raoult's law can be used to determine the molar mass of an unknown substance based on the vapor pressure of a solution. This problem will walk you through the steps required.
A chemist wants to determine the molar mass of a naturally occurring substance isolated from spearmint. She adds 4.231 g of this substance to 25.00 g of acetone (C3H6O; molar mass 58.08 g/mol). The vapor pressure of the solution is determined to be 0.2474 atm at 25°C. If the vapor pressure of pure acetone at this temperature is 0.2630 atm, what is the mole fraction of the solvent in this solution? Give your answer to four significant digits.
Using your answer from Part 1, calculate the moles of solute present in the solution. Give your answer to three significant digits.
Finally you can calculate the molar mass of the solute. Remember that 4.231 g of solute was added to 25.0 g of acetone, giving a solution vapor pressure of 0.2474 atm.
Use the mass of solute and your answer to Part 2 to calculate the molar mass, giving your answer to three significant digits.
Explanation / Answer
i) According to Raoult's law
Psolution = XsolventP°solvent
Psolution = Vapor pressure of solution, 0.2474atm
Xsolvent = mole fraction of solvent in liquid
P°solvent = Vapor pressure of solvent in pure form, 0.2630atm
Xsolvent = Psolution/P°solvent
= 0.2474atm/0.2630atm
= 0.9407
mole fraction of solvent = 0.941
mole fraction of unknown substance (solute) = 1 - 0.941 = 0.0590
ii) mole fraction = no of mole/ total no of mole
no of mole of acetone = 25g/58.08g/mol = 0.43045
Total no of mole = 0.43045/0.941 = 0.45744
No of mole of solute(unknown sbstance) = total no of mole × mole fraction of solute
= 0.45744 × 0.0590
= 0.0270mole
iii) molar mass of solute = mass of solute /no of mole of solute
= 4.231g /0.0270mol
= 157g/mol
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