To find the mass of a substance when given the number of moles and the molar mass, you can use the formula: mass = number of moles × molar mass. In this case, the number of moles of nitrogen dioxide (NO₂) is 3.45 moles, and the molar mass of NO₂ is 46.01 g/mol. Therefore, the calculation is as follows: 3.45 moles × 46.01 g/mol = 158.73 g. This means that 3.45 moles of nitrogen dioxide has a mass of 158.73 grams.