What is the empirical formula of a compound containing 40.0% sulfur and 60.0% oxygen by mass?

To find the empirical formula of a compound, follow these steps:

1. Assume you have 100g of the compound. This means you have 40g of sulfur (S) and 60g of oxygen (O).

2. Convert the mass of each element to moles. The atomic mass of S is approximately 32g/mol and that of O is approximately 16g/mol. So, you have 40g/32g/mol = 1.25 mol of S and 60g/16g/mol = 3.75 mol of O.

3. Write the ratio of the moles. This gives you the formula S1.25O3.75.

4. To get the empirical formula, you need to find the simplest whole number ratio. Divide each of the subscripts by the smallest number of moles calculated (in this case, 1.25). This gives you S1O3.

So, the empirical formula of the compound is SO3.