What is the mass in grams of 4.2 moles of FeO2?

To find the mass of 4.2 moles of FeO2, first calculate the molar mass of FeO2. Iron (Fe) has a molar mass of approximately 55.85 g/mol, and oxygen (O) has a molar mass of about 16.00 g/mol. Therefore, the molar mass of FeO2 is 55.85 g/mol + (2 × 16.00 g/mol) = 87.85 g/mol. Multiplying the molar mass by the number of moles gives: 4.2 moles × 87.85 g/mol = 369.07 grams.