Molar Mass of a Volatile Liquid by the Dumas Method


Background: ideal gas law

A volatile liquid (or solid, for that matter) is one which evaporates easily. [Note: volatile is not a synonym of reactive.] We will evaporate the liquid in this lab, and make use of the properties of the resulting vapor (gas).

From the 17th through the early 19th centuries, a number of scientists discovered simple relationships among the temperature, pressure, volume, and amounts of gases:

These relationships can be combined into a single equation which describes the relationship among temperature, pressure, volume, and amount (number of moles) of all gases under ordinary conditions. The equation is the ideal gas law:
pV = nRT ,

The ideal gas law has wide applicability in chemistry. One can use it to compute any one of the four variables if the other three are known or can be measured. In this week's experiment, we will determine the pressure, volume, and temperature of a sample of gas (our evaporated volatile liquid), which means that we can solve for the number of moles:

number of moles = n = pV/RT .

But we go one step further, because we will also measure the mass of the evaporated liquid. Knowing the mass and the number of moles will allow us to compute the molar mass of the liquid:
molar mass = (mass)/(number of moles).

Procedure Overview

I won't go over the procedure in step-by-step detail, but I will stress some points of safety and (in bold color) some places where our procedure differs from that in the lab packet. The main difference is to measure the volume of the flask last, not first.

Work in pairs (no triples).

We will heat a volatile liquid to evaporate it so that its vapor just fills a flask whose volume we can measure; we will also know the temperature and pressure of the vapor, and the mass of the vapor.

  1. Determine the mass of flask, boiling stone, and foil cap
  2. Add volatile liquid
  3. Heat to vaporize liquid
  4. Measure water bath temperature and atmospheric pressure
  5. Condense vapor in flask
  6. Determine mass of flask, boiling stone, foil cap, and condensed vapor
  7. Determine volume of flask The flasks are nominally 250 mL, but 250 mL is not an exact measurement; furthermore, it refers to the volume of liquid it holds in normal use, so it excludes the volume of the flask's neck.