Passage IV (Questions 19-23)

A student wants to know the molecular weight of a substance that is liquid at room temperature but a gas at 100° C.

She heats the substance until it vaporizes, then fills an open 400-mL flask with the gas and maintains the temperature at 100° C. She then cools the flask to room temperature. After the vapor in the flask condenses, she finds that this sample weighs 0.900 g. The barometric pressure is 750 torr.

The student then calculates that the number of moles of gas in the sample is 0.0129. She further calculates that the molecular weight of the sample must be 72.1 g/mol.

However, her instructor tells her that the molecular weight of the sample should in fact be 24.0 g/mol.