1 AP Chemistry Lab Brockport High School NY USA Molecular Weight of a Condensable Vapor Mr Keefer Introduction The molecular weight of a condensable vapor can be determined by adding an excess of the corresponding fluid to a weighed flask, heating the assembly to volatilize all the liquid and to expel the excess vapor, cooling, and then weighing the flask with the condensed liquid. The mass of the condensed liquid plus the mass of vapor left uncondensed at room temperature is equal to the mass of vapor that fills the flask at the elevated temperature and the prevailing pressure. If the liquid is relatively nonvolatile at room temperature, then we can neglect the mass left uncondensed at room temperature. Given the mass of the vapor that fills the flask at the elevated temperature, we can calculate from the gas laws the volume of the measured mass of vapor at STP. Since at STP 1 mole of ideal gas occupies 22.4 L, we can calculate the number of moles present and hence the mass per mole, or molecular weight. Materials and Equipment
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