Factual error: The film takes place in a private boarding school the 1950's, however one of the students is studying from a 1980's chemistry textbook. The obvious giveaway is that the spine of one book shows HBJ as the publisher. Jovanovich was not added to Harcourt Brace until the 1970s.
Factual error: Keating misquotes Whitman's "Song of Myself". The word "rooftops" should have been "roofs". An English Lit teacher of his caliber would not have made this mistake. This is a deliberate mistake on the part of the filmmakers.
Chosen answer: He is just trying to get something to help wash the taste of vomit out of Todd's mouth.
Macalou