- The chemical formula for sodium sulfate is Na2SO4.

- Each formula unit of Na2SO4 contains 1 sulfate ion (SO4^2-).

- Each sulfate ion contains 4 oxygen atoms (O).

- Since there are 3 formula units of Na2SO4, there are a total of 3 x 4 = 12 oxygen atoms.

Therefore, there are 12 atoms of O in 3Na2SO4.