If a three-carbon alkene contains two double bonds. predict it's molecular formula?

H2 - C =C = C - H2

Explanation:

In general a carbon has a valency of four thus it need to make four covalent  (as carbon is non-ionic) bonds in order to fulfill its valence shell.

Now alkene consist of two carbon atoms. When these two carbon atoms form double bond , the requirement of 2 covalent bond gets fulfilled, thus each carbon now need two more bonds to become stable.

In lie to this requirement each carbon atom makes single bond with two hydrogen atoms and so the formula of this compound becomes

H2 - C =C= C - H2

The valency of central carbon atom containing two double bonds is balanced by covalent bonding between its own electron and the electron shared by the other two carbon atoms.