Question

The molecular mass of the compound is 90 amu. What is the molecular formula?

Answer 1

If you're asking what I think you are, then it is as easy as looking at a periodic table and seeing what element has a mass number of 80amu.

Answer 2

If compound contains more than one element, you should
Know which elements.

Answer 3

90 amu*

Answer 4

Also Geb is right. With the way your question is worded it could be any number of compounds. I just thought you meant element.