I've graphed a distillation curve (temperature against volume of distillate) from the data obtained from a simple distillation experiment. The substance can be any of methanol, ethanol, 1-propanol or a 1-propanol and water azeotrope).

How can I find the boiling point from this curve to determine the identity of the compound?

I should also mention that there was leftover nonvolatile dye in the round bottom flask.


Do you mean that the substance could be pure methanol, ethanol or 1-propanol?

In that case there is a simple way to distinguish between the four options. A pure substance will have a T-V curve that consists of 2 straight lines, 1 running from $T=T_{start}$ to $T=T_{b}$ at $V=0$ and one running from $V=0$ to $V=V_{total}$ at $T=T_{b}$. Probably you will have some impurities which cause some small deviations from this perfect hook, but that will be the general shape. Determining the boiling temperature is just a matter of checking where the curve goes straight up. Depending on the temperature at which this happens you can identify methanol ($T_{b}\approx65$), ethanol ($T_{b}\approx78$) or 1-propanol ($T_{b}\approx97$).

The azeotropic mixture makes it a bit tricky, because the boiling points of 1-propanol and water are so close together, but the curve should show a distinct difference not going vertically up at a certain temperature.

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  • $\begingroup$ If it is a straight line, does that mean that Tstart = Tb? $\endgroup$ – Adi Oct 10 '13 at 14:08
  • $\begingroup$ No, it means that $V=0$ at $T_{start}$ and $T_b$ $\endgroup$ – Michiel Oct 11 '13 at 5:09

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