The percent error in molar mass cannot be calculated without the actual measured volume. The molar mass calculation is sensitive to the accuracy of the volume measurement, and an assumed volume that deviates from the true volume would result in an incorrect molar mass.
The question revolves around the concept of the molar mass of a gas and how an error in volume measurement can affect the calculation of that molar mass. First, to address the provided question about the percent error, we would need the actual measured volume to compare it with the assumed volume of 125 mL. Without this information, it is impossible to calculate the percent error. However, assuming the mention of molar mass = 75.5 is meant to indicate the correct molar mass calculated with the true volume, we can discuss how an incorrect volume assumption affects the calculated molar mass.
Regarding the calculation of molar mass, it can be determined from the given conditions using the Ideal Gas Law, which relates pressure, volume, temperature, and the amount of gas in moles. The exact calculation would involve converting the measured volume to liters, the temperature to Kelvin, and the pressure to atmospheres, and then using the Ideal Gas Law formula PV = nRT to solve for n, the number of moles of the gas. After finding the number of moles, the molar mass (M) is found by dividing the mass of the gas by the number of moles: M = mass/n. In this case, a deviation from the correct volume would lead to an incorrect determination of the number of moles and, consequently, an inaccurate molar mass.