Why were and why not Was

[Forgmat]

the following sentence is grammatically correct:

Fifty millilitres of sodium chloride solution were added in the second step of the experiment.

However can someone please make me understand as to why " was " cannot be used in place of " were "in the above sentence

thanks

[Yankee]

Welcome to the forum, Forgmat.

Was your sentence a test question?

In my opinion there is nothing wrong with using 'was' in your sentence. Actually, I think either could be used. Things such as a measured distance, a specific quantity, a specific amount of money, or a period of years, when seen as a single unit, can be used with a singular verb.

Compare these:
Twenty-six miles is the usual length of a marathon.
Fifty dollars is too much to pay for that.
Mr. Johnson's class is really boring. Two hours seems like an eternity in his class.