Question: how many oxygen atoms are in a formula unit of sodium sulfate?
Answer: The formula mass for sodium sulfate was calculated on page T-9. Referring to those numbers, the formula Na2SO4 means: two moles sodium (45.98 g), one mole sulfur (32.06 g), and four moles oxygen (64.00 g) combine to form one mole of sodium sulfate (142.04 g).