In the reaction C + O2 → CO2, 18 g of carbon react with oxygen to produce 72 g of carbon dioxide. What mass of oxygen would be needed in the reaction?

By conservation of mass, the mass of the reactants equals the mass of the products. The carbon dioxide has a mass of 72 g, and 18 g of that mass comes from carbon, so the oxygen mass is 72 − 18 = 54 g. The correct choice is 54 g.

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