Final answer:
Without additional details such as the volume of the flask and the pressure and temperature conditions, the mass of the dry air in the sealed flask cannot be calculated. The question lacks sufficient data to provide a definitive answer.
Step-by-step explanation:
To calculate the mass of the dry air that was in the sealed flask, we need more information than what is provided in the question alone. The mass of air depends on its volume, pressure, temperature, and the molar mass of air, which can be determined using the ideal gas law in conjunction with Avogadro's law or other relevant chemical properties. Without additional information such as the volume of the flask, the pressure and temperature at which the flask was sealed, and the molar mass or composition of the air in the flask, we cannot determine the mass of the air within it.
However, based on the information provided, the average molar mass of dry air can be calculated using the given density at 1 atm and 20 °C. Using the equation for density (density = mass/volume), we can rearrange to find the molar mass (mass = density × molar volume). Since the molar volume of a gas at standard temperature and pressure is 22.4 L, and the given density is 1.2256 g/L, the molar mass of dry air can be calculated.