In an experiment a piece of magnesium ribbon is burned in air in a crucible. the mass of the mixture of MgO and magnesium nitride after burning is 0.470g. Water is added to the crucible, further reaction occurs, and the crucible is heated to dryness until the final product is 0.486 g of MgO. What was the mass percentsge of magnesium nitride in the mixture obtained after initial burning?