1. Identify the elements and their atomic masses:

* Carbon (C): 12.011 amu (atomic mass units)

* Oxygen (O): 15.999 amu

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

* Carbon: 12.011 amu * 1 = 12.011 amu

* Oxygen: 15.999 amu * 2 = 31.998 amu

3. Add the results together:

* 12.011 amu + 31.998 amu = 44.009 amu

Therefore, the formula mass of CO2 is 44.009 amu.