Molecular Weight of a Volatile Compound


IntroductionThe ideal gas law serves as the basis for the Dumas Method for the determination of molecular weight. According to the ideal gas law

PV = nRT

where P, V, n, R and T are the pressure in atm, the volume in liters, the moles of gas, the gas constant (equal to 0.08206 l-atm/(K mole) and the absolute temperature, respectively. Letting n = m/(MW) where m is the mass of gas grams and MW is its molecular weight, the ideal gas law gives

MW =mRT/(PV) (1)

Thus, the molecular weight of an ideal gas can be calculated if its mass, pressure, temperature, and volume are known.

In the Dumas Method a small volume of the liquid compound whose molecular weight is to be determined is placed in a flask of known mass and volume. The flask has a small opening to the atmosphere. See Figure (1). Heating the flask and contents in a boiling water bath causes the liquid to vaporize. The resulting vapor fills the flask, driving air out through the hole. Excess vapor escapes until the pressure of the vapor inside the flask equals that of the atmosphere outside. The flask now contains pure vapor at atmospheric pressure. The small hole reduces the diffusion of air into the flask.

If the vapor in the flask is assumed to be an ideal gas, equation (1) can be used to determine its molecular weight. In equation (1):

The temperature of the vapor is that of the boiling water bath.

The pressure of the vapor is that of the atmosphere.

The volume of the vapor is that of the flask and glass tube.

The mass of the vapor is determined by removing the flask from the waterbath, allowing it to cool, weighing the flask and condensed liquid, and subtracting the mass of the empty flask.

Caution: WEAR EYE PROTECTION throughout this experiment.

Stockroom: Things to borrow and return on the same day.

Procedure: Bend a disposable pipet by heating it in a flame until soft. Be careful that you do not close off the pipet at the bend. Let the glass cool, lubricate it with glycerol and carefully insert it into a rubber stopper. Hold the glass and stopper with paper towels so if the glass breaks, you do not get cut. See Figure 1. Determine and record the mass of a clean, dry 250 ml flask along with the stopper and bent glass tube. Be certain the balance reads zero before all weighings.

Record the identification number of your unknown liquid and place about 3 ml of the unknown into the flask.

Clamp the flask to a ring stand, and completely immerse it in a 1000 ml beaker that is on a hot plate. Fill the beaker with water so that as much of the flask as possible is immersed in the water. Make a canopy from aluminum foil to cover the top of the flask and as much of the pipet as possible. See Figure 2. 

Take out your flask and boil the water using a hot plate (NOT the bunsen burner.)  Add boiling chips to the water to prevent bumping.

 Add about 3 mL of your unknown to the flask and place the flask in the water bath as shown in Figure 2. Then, resume heating to bring the water to a boil, and measure the temperature of the bath. Observe the plume coming from the pipet closely in order to decide when all of the liquid has evaporated. Since the escaping vapor and the surrounding air have different indices of refraction, it is possible to see the vapor streaming out of the hole. Again, exercise due care to avoid being spattered by the boiling water. Continue heating for about 1 minute after you can no longer see the escaping vapor.

Remove the flask from the water bath by loosening the clamp from the supporting rod and moving it upward. Allow the flask to cool to room temperature, and dry it gently and thoroughly with a paper towel. Weigh the flask, stopper, pipet and condensed liquid.

Measure the atmospheric pressure with the barometer in the laboratory. Barometer instructions.

Do the experiment in triplicate. After you have completed three trials, measure the volume of the flask by filling it with water and measuring the volume of water with a graduated cylinder. Measure the volume of the pipet by also filling it with water and measuring the volume of water it holds. A single volume measurement is adequate.

Waste: Dispose of excess unknown in the solvent waste container and discard the empty vials in the trash.

ResultsRecord all data for the three trials. Use the table below as a guide for your data collection and calculations. Please show all calculations clearly.

Unknown number
Mass of flask, stopper, pipet and condensed vapor
Mass of flask, stopper and pipet
Temp. of boiling water
Barometric pressure
Mass of condensed vapor
Volume of flask
Volume of pipet
Total volume of vapor
Average total volume of vapor
Temperature of vapor
Molecular weight of unknown
Average molecular weight of unknown

ConclusionReport the average molecular weight and its average deviation.

Question:  What are some possible sources of error in this experiment?