Why does ch3coch3 has stronger intermolecular forces than c2h5oc2h5? even though both have dipole-dipole as their imf, but c2h5oc2h5 has a larger molecular weight and as the molecular weight increases, the imf get stronger. so why it is the opposite here?

I think because the CH3COCH3(second picture) has a ketone(the double bond to oxygen C=O) in the chemical structure and I believe ketones lead to stronger IMF than ethers(C-O-C). I am not sure about this, but I hope this answer will at least help give you a hint as to what to look up to figure this out. I'm sorry I couldn't be of more help! 

Yes i think so not 100 percent sure though

The answer to this problem is. 67