Problem: An unknown compound was determined to be 74.97% carbon, 8.39% hydrogen, and 16.64% oxygen. Calculate the empirical formula.

We’re being asked to determine the empirical formula of an unknown compound given the mass percent of C, H, and O.

Recall that the empirical formula is the simplest whole-number ratio formula of a compound. 

Step 1: Recall that mass percent is given by:

$\boxed{\mathbf{\%}\mathbf{ }\mathbf{mass}\mathbf{=}\frac{\mathbf{mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{X}}{\mathbf{total}\mathbf{ }\mathbf{mass}}\mathbf{\times }\mathbf{100}}$

The problem states that the compound is 74.97% C, 8.39 % H, and 16.64% O. Assuming we have 100 g of the compound, this means we have 74.97 g C, 8.39 H, and 16.64 g O.

Now, we need to get the moles of each element in the compound. 

The atomic masses are 12.01 g/mol C, 1.01 g/mol H, and 16.00 g/mol O.