Question

a three carbon alkene contains two double bond predict its molecular formula ? dont know this ancer

Answer 1

If you have three carbons, they must be bonded to each other with two bonds. If you have two double bonds, they must be the bonds between the carbons, because H can't form double bonds. So you know your skeleton must look like this:

Now, knowing how many bonds each carbon atom wants to form, and how many each hydrogen atom can form, how many hydrogens do you need to complete the molecule?