Question

A three-carbon alkene contains two double bonds. Predict its molecular formula?

Answer 1

It would help if you can visualize the molecule first. There are three carbon atoms, bonded together through double bonds. Each carbon must have four bonds connected to it in order to be neutral in terms of charge. If I were you, I would first draw out the three carbons:

Now, we see that the second carbon (in the middle) has four bonds already, so it's alright. But then the two other carbon atoms only have two bonds. So we need to add hydrogen atoms to them, so that the entire alkene is neutral in charge.

So from the molecular structure, you can deduce the molecular formula.