Sodium can be reacted with oxygen at 130–200°C to produce sodium oxide (Na₂O, white powder) and sodium peroxide (Na₂O₂, yellow solid). Sodium oxide can further absorb oxygen to produce sodium peroxide. A piece of sodium metal was reacted with oxygen and produced a mixture of 0.496 g of white and yellow solid. The mixture was further exposed to oxygen until the final product is pure yellow solid with a mass of 0.525 g. Calculate the mass percentage of sodium oxide in the initial mixture.