1. Look up the atomic masses of oxygen (O) and fluorine (F):

* Oxygen (O): 16.00 g/mol

* Fluorine (F): 19.00 g/mol

2. Multiply the atomic mass of each element by its subscript in the formula:

* Oxygen (O): 16.00 g/mol * 5 = 80.00 g/mol

* Fluorine (F): 19.00 g/mol * 2 = 38.00 g/mol

3. Add the results from step 2 to find the formula mass:

* 80.00 g/mol + 38.00 g/mol = 118.00 g/mol

Therefore, the formula mass of O5F2 is 118.00 g/mol.