The molar mass of a volatile liquid (bp < 90˚C) is to be determined by measuring the density of its — Laboratory Chemistry Chemistry Question

The molar mass of a volatile liquid (bp < 90˚C) is to be determined by measuring the density of its vapor in an Erlenmeyer flask capped with Al foil with a pinhole to allow the vapor to escape. A sample of the liquid is added to the pre-weighed flask that is heated in a water bath until the liquid has evaporated, after which the flask is dried and reweighed. Which piece of information is not required to determine the molar mass of the liquid?