Methanol and ethanol form an almost ideal solution…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Methanol and ethanol form an almost ideal solution. At the vapour pressures of the pure liquids are for methanol and for ethanol. A solution is made from of methanol, with molar mass , and of ethanol, with molar mass . Find the mole fraction of methanol in the liquid, the total vapour pressure and the mole fraction of methanol in the vapour.
Need a reminder? The key ideaShow
For two volatile liquids, Raoult's law gives each partial pressure as , and the total pressure is the sum, . The vapour has . For a non-volatile solute, and . Mole fractions use moles, so divide each mass by its molar mass first.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Masses of and , with the molar masses written in the question.
Plan. Masses to moles, moles to mole fractions, then Raoult's law and Dalton's law.
Step 1. Moles. and .
Step 2. Mole fraction in the liquid. .
Step 3. Partial pressures. . .
Step 4. Total. .
Step 5. The vapour. .
Answer: , and .
Watch out. The masses are in the ratio , but the moles are in the ratio . Using masses in place of moles gives a wrong mole fraction.
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