Question

no. particles in 8gms of O2 molecules

Answer 1

Given:
N =?
m = 8g
M = 16*2 = 32

no. of particles = given mass/molar mass* avagadro's no.
or
N = m/M* N0
N = 8/32 * 6.022 * 10 to the power 23
   = 1.5055 * 10 to the power 23
   = 1.52 * 10 to the power 23 => answer

HOPE THIS WILL BE HELPFUL  TO U....  :-]

Answer 2

Are you asking number of molecules? If yes, then here is the answer

Molar mass of O2 is 32 grams
32 grams of O2 contains Avagadro number of molecules

So, 8 gram contains
= 8/32 × Avagadro number
= 0.25 × Avagadro number
= 0.25 × 6.022 × 10^23
= 1.5055 × 10^23 molecules