A student makes a compound of sulfur and oxygen. She uses 5.00 g of sulfur and 4.99 g of oxygen, and all of the elemental substances are completely used up. What is the percent sulfur in the compound?

First, we apply the law of conservation of mass which states that the total mass in a system remains constant.

Therefore, there must be 5.00 g of sulfur and 4.99 g of oxygen in the product. Now, we determine the mass percentage using:

Mass % = (mass of sulfur x 100) / total mass of compound

Mass % = (5 * 100) / (5 + 4.99)

Mass % = 50.05%

The product contains 50.05% sulfur by mass.