If there are 20.0 grams of oxygen in a sample of sulfurous acid then how many moles of sulfurous acids are there in the sample? I really need this problem worked out, not just the answer. Thank you.

Sulfurous acid is H2SO3
Calculate moles of O in 20 g --- 20g of O / 16 g /mole = 1.25 moles
now 1mole of H2SO3 = 3  moles of O
so moles of H2SO3 = 1.25 moles X 1mole of H2SO3 / 3moles of O = 0.417 mole