Question

Adil knows that he has one of 3 possible liquid unknowns: tetrahyrofuran (bp 65-67°C), ethyl acetate (bp 76-77°C), or toluene (bp 110-111°C). He sets up for a microscale bp determination and clamps the whole shebang in a water bath. Fifteen minutes late the water is boiling like crazy, but he has yet to see a single vapor bubble escape from the capillary tube. He double-checks his set-up and everything is fine so now he’s totally confused. What must his unknown be and why? Break it to him gently…he doesn’t want to feel like a totally lost.

Answer 1

Water is boiling like crazy at normal atmospheric pressure means the temperature has reached 100 ^oC. We know, temperature does not change during phase transition (like liquid to vapour). So, as long as water is boiling off and vapourizes, the temperature is fixed at 100 ^oC.

If your sample contained tetrahydrofuran(THF) then it would boil at temperature 65-67 ^oC i.e, at warm water, not even boiling condition of water. So, it is easy to observe the boiling of your sample. Since, you haven't seen it, your sample doesn't contain THF.

If your sample contained ethyl acetate then it would boil at temperature 76-77 ^oC i.e, even more warm water, although not even boiling condition of water. So, it is again easy to observe the boiling of your sample. Again, you haven't seen it, so, your sample doesn't contain ethyl acetate.

So, your sample must contains toluene. But, why aren't you be able to see it ?

The reason I stated at first. Since, water is boiling, the temperature is fixed at 100 ^oC. But your sample is supposed to boil at 110-111 ^oC. So, you can never see the boiling of your sample in water bath because it will never supply the heat needed for your sample to boil.