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! 

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.