Question

Calculate no. Of mass in 3.011×10(23) no. Of O2

Answer 1

If you mean Oxygen as atoms, its molar mass is 16 g/mol, if you mean moleceule (O2) it is 32 g/mol.
I assume you meant atoms, so let’s stick with 16 g/mol.
Molar mass tells us about the weight of one mole of something. And one mole is 6.022 * 10^23 atmos/molecules (Avogadro's number). So 6.022 * 10^23 Oxygen atoms is 16 g (1 mole * molar mass).
Now you can use proportion: 
6.022 * 10^23 at. = 16 g
3.011 * 10^23 at. = x g
So x = 8g
Or you can check, how many moles is 3.011*10^23 at. - and it is 1/2, then just multiplay moles * molar mass, so you get
1/2 mol * 16 g/mol = 8 g