How many grams of O are in 735 g of Na2O

method #1
1mole Na2O contains 1mole of O
735g / 62g/mole = 11.85moles Na2O so 11.85moles O = 189.67g

method #2
the mass of O = ?% of Na2O
16g / 62g x 100% = 25.81%
735g x 0.2581 = 189.67g