Hello, I would like to know which one of these two sentences is the correct one:
a) 9 mL of phosphoric acid were added to the flask
b) 9 mL of phosphoric acid was added to the flask
Thanks in advance.
First lets get the metric symbol correct. The symbol (and please don't call it an abbreviation because it is not) for millilitre is ml; the symbol for millilambert is mL. You can't pour Lamberts.
Now to the point of your question. Both are correct. It's in how you think of the operation of adding the milliltres. If you added them one ml at a time, then you would use the plural; if you added them in one pour, then you use the singular, because the sentence really says "[A quantity of]9 ml of liquid was added" - the implied subject is "a quantity" so it uses the singular.
If the result sounds so odd that you just can't abide the dilemma, then recast the sentence: The scientist then added 9ml of acid to the flask. It's a better construction anyway because it is not in the passive. The scientist added is active; 9 ml of acid was/were added is passive (and therefore generally less interesting).