Ethanoic acid is also known as acetic acid.
Dilute aqueous solutions are called vinegar.
Butanoic acid is also known as butyric acid.
All data from CRC Handbook.
This leads to a whole package of riddles.
First, suppose I had showed you this table with
the two starred items not filled in. Would you
have been able to predict anything close to the
observed answer?
Note that adding some water to formic acid raises
the boiling point above the BP of either constituent.
For the propanoic and butanoic acids, the BP of
the azeotrope is within a fraction of a degree of
the dominant constituent (water).
So why is ethanoic acid so different? The azeotrope
contains only a small percentage of acid. Adding
a tiny amount of a high-boiling substance lowers
the BP more than 23 degrees.
To put it in simplest terms, what is going on here?
I have no clue. I've rummaged around. I've found
a number of "just-so stories" that allegedly explain
the boiling of mixtures, but for every correct
prediction they make multiple incorrect predictions,
so they can't be taken seriously as hypotheses.
I even entertained the hypothesis was that it might
be a misprint ... 96.6 would have been more plausible
than 76.6 ... so I did the experiment. It's a rather
simple experiment: pot of vinegar plus thermometer
plus small amount of patience. It ain't a misprint.