Question

what do you mean by saying that molecular mass of oxygen is 32

Answer 1

in the atmosphere oxygen exists as O2 where two oxygen atoms bond to make a stable molecule. Now one atom of oxygen has atomic weight 16, so two oxygen molecules will have 16+16=32 molecular mass. 

Answer 2

Oxygen in its molecular form exists as O2.

One oxygen atom has molecular mass = 16 so,

1 molecule of the oxygen will have molecular

mass = 2 x 16 =  32 . Hope this helps you .