Question

C + O2 → CO2

(M O2 = 32 g/mol; M CO2 = 44 g/mol)

When 78.0 g of carbon dioxide (CO2) are produced, what mass of oxygen is present in the product?

Answer 1

78.0 g CO2/44 = 1.77 mols CO2

mass O2 required = 1.77 mols CO2 x (1 mol O2/1 mol CO2) = 1.77 mols O2.
Then 1.77 mols O2 x molar mass O2 = 1.77 x 32 = ? g O2.

Another way.
% O in CO2 = (32/44)*100 = 72.72% oxygen
Then 78.0 x 0.7272 = ?

Another way.
78 x (2*atomic mass O/molar mass CO2) = ?