Dumas Bulb For Vapour Density

Quick Answer: A Dumas bulb is a small, thin glass bulb with a long, narrow capillary neck, used in the Dumas method to find the molar mass of a volatile liquid from the mass of its vapour at a known temperature and pressure.

The Dumas Method Step by Step

Jean-Baptiste Dumas devised this method in the 1820s, and it remains a standard physical chemistry practical. The clean, dry bulb is weighed with its capillary open. A few millilitres of a volatile liquid, such as propanone or a low-boiling hydrocarbon, are added, and the bulb is held in a boiling water bath. The liquid vaporises and its vapour sweeps the air out through the capillary; when no liquid remains, the bulb is full of vapour at the bath temperature and atmospheric pressure.

The bulb is then removed, the vapour condenses as it cools, and the bulb is dried and weighed again; the gain in mass is the mass of the vapour. Filling the bulb with water and weighing it gives its internal volume. The molar mass then follows from the ideal gas equation, M = mRT / PV, using the bath temperature and the barometric pressure. In the classic procedure the capillary tip is sealed in a flame while the bulb is still hot, so no vapour is lost; simplified class versions leave the tip open and weigh quickly, accepting a small error.

Because the bulb is thin and heated, it must be handled gently and never heated with its tip sealed while liquid is still inside. The listing does not state capacity or glass type. For measuring the density of a liquid rather than a vapour, the pyknometer is the matching instrument.

Specifications

Item	Dumas bulb for vapour density and molar mass determination
Form	Thin glass bulb with a long capillary neck
Method	Vaporise, weigh the vapour, calculate with M = mRT/PV
Suited to	Volatile liquids that boil well below the bath temperature
Capacity and glass type	Confirm at enquiry

Care & Handling

Heat the bulb in a water bath inside a fume cupboard, and if the tip is to be flame-sealed, do it only once all liquid has vaporised and with solvent bottles well away.
Handle the capillary neck lightly; it is the most fragile part and breaks easily in a clamp.
Dry the outside of the bulb completely before each weighing, since bath water on the glass adds mass.
Rinse with a little propanone and let it dry with the tip open before reuse, making sure no liquid remains inside.

Applications

Physical chemistry practicals determining the molar mass of a volatile liquid
Teaching the ideal gas equation through a real measurement and its error analysis
Comparing a measured molar mass with formula masses to identify an unknown liquid

Why Choose LabEquip

University and college physical chemistry laboratories use Dumas bulbs for molar mass practicals alongside their gas-law teaching. The item is part of LabEquip’s chemistry lab equipment; send quantities through the contact page.

Frequently Asked Questions

Why is the Dumas bulb heated in a water bath rather than over a flame?

A boiling water bath holds the bulb at a steady, known temperature, which the calculation needs, and heats the thin glass evenly. A direct flame gives an uncertain vapour temperature and can crack the bulb or ignite a flammable vapour.

Which liquids suit the Dumas method?

Liquids that boil well below the temperature of the heating bath, so they vaporise completely, such as propanone, ethanol or low-boiling hydrocarbons with a boiling water bath. The liquid must not decompose on heating.

How is the molar mass calculated?

Use M = mRT / PV, where m is the mass of vapour, T the bath temperature in kelvin, P the atmospheric pressure, V the internal volume of the bulb and R the gas constant. Keep the units consistent, such as pascals and cubic metres.

Why is my result higher than the true molar mass?

Real vapours close to their boiling point are denser than the ideal gas equation predicts, and water drops on the outside or unvaporised liquid inside add mass. Each of these errors usually pushes the result up.

How do I find the volume of the bulb?

After the vapour measurement, fill the bulb completely with water, weigh it, and divide the mass of water by its density at room temperature. Alternatively, empty the water into a measuring cylinder and read the volume.

Is it safe to heat a sealed Dumas bulb?

Never heat the bulb with its tip sealed while liquid remains inside, because the vapour pressure can burst it. The tip is sealed, if at all, only after all the liquid has vaporised and the excess vapour has escaped.

Last Updated: September 2026