Not a limerick, though it has a similar feel. The way I originally heard it was:

    Once there was a chemist
    There isn't any more
    What he thought was H2O
    Was H2SO4.

I've heard a few variants, such as "Bobby was a chemist" or "There isn't one no more". At least one blog[1] claims the original poem from 1894 is:

    Our Willie passed away today,
    His face we’ll see no more;
    What Willie thought was H2O
    Proved H2SO4.

[1] https://www.barrypopik.com/index.php/new_york_city/entry/joh...