6.5L of the final volume <was><were> used

[Falesia]

Hello,

I'd really appreciate if someone could provide an explanation for the following:

Which of the phrases is correct?

" ... 6.5L of the final volume was used..." or "... 6.5L of the final volume were used..."

What is the subject, 6.5L or final volume? To which should the verb relate to?

Thanks in advance!

 

[Mister Micawber]

.
The subject is 6.5 liters, and you can consider that either as a single lump quantity or as several countable liters. Either verb is grammatically acceptable.
.

 

[Falesia]

Thanks for this!