a laboratory group

Question

A laboratory group measures an excess of a volatile liquid. They add this to an empty flask with a volume of 705 mL. They heat the sample to 88.5 degrees Celcius until all the liquid is vaporized. The pressure in the lab is 0.987 atm. If 0.256 grams of volatile liquid were left in the flask, what was the molar mass of the compound? 

Answer 1

From the information provided, we can use the ideal gas law to find the moles of gas but in order to find the molar mass of this compound, we need to know the mass. Since we are not given the mass of excess volatile liquid, but only given the mass of the liquid left over, we cannot solve for molar mass. If you have the original mass (excess) of the volatile liquid, then use PV = nRT and solve for n.

n = PV/RT = (0.987 atm)(0.705 L) / (0.0821 Latm/Kmol)(362K)

n = 0.0234 moles

Then divide 0.0234 moles by mass of liquid, i.e. original mass - 0.256 g to find molar mass of unknown