Breakdown of Он выпил около полутора литров воды.
вода
water
он
he / it (for a masculine noun) / it (for a neuter noun)
выпить
to drink (perfective)
около
near
Questions & Answers about Он выпил около полутора литров воды.
Why is it полутора instead of полтора?
The preposition около (about) requires the Genitive case. While полтора is used for the Nominative and inanimate Accusative cases, its form for all other cases (the "oblique" cases) is полутора.
I learned that 1.5 takes the Genitive singular. Why does this sentence use the Genitive plural литров?
You are right that in the Nominative and Accusative cases, we use the Genitive singular (полтора литра). But when the number goes into an oblique case like the Genitive полутора used here, the noun stops using that special rule and simply matches the case in the plural. Therefore, we must use the Genitive plural литров.
If the noun being measured was feminine, would полутора change form?
No. In the Nominative and Accusative cases, 1.5 has different forms for masculine/neuter (полтора) and feminine (полторы). However, in all other cases, they share the exact same form: полутора.
Why are there two Genitive words in a row (литров воды)?
Литров is in the Genitive plural because it is grammatically tied to the oblique number полутора. Воды is in the Genitive singular because it describes the measurement ("liters of water"). Russian frequently chains Genitive nouns like this to show "of" relationships.