Question

one element X having equivalent weight 8 and valency 2 then find its atomic weight ?

Answer 1

The problem was that physics stuck to isotopically pure O-16 for their mass scale, and chemistry used "oxygen" (i.e., with some O-17 and O-18 present). There's 0.028% difference between those scales. That may not sound like much, but with molecules of ~200-500 amu, it starts to add up.