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A series of measurements are made in order to determine the molar mass of an unknown gas. First, a large flask is evacuated and found to weigh 134.567 g. It is then filled with the gas to a pressure of 735 torr at 31°C and reweighed; its mass is now 137.456 g. Finally, the flask is filled with water at 31°C and found to weigh 1067.9g. The density of water at this temperature is 0.997 g/mL. Assuming that the ideal-gas equation applies, what is the molar mass of the unknown gas?

The difference in weight tells you the mass M of the gas in the flask. The water filling tells you volume V of the flask. You are told what the gas pressure P and temperature T are.

PV = nRT tell you the number of moles n of the gas. R is the molar gas cosntant.

The molar mass is M/n.

Crank out the numbers.

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