Name: Law of Multiple Proportions
Uploaded: 2019-07-19
Duration: 6 min 43 s
Description: The Law of multiple proportions is applicable only when two elements combine to form two or more compounds. Watch this video to know how this law is applicable.

Question

Define vapour density. How will you determine the vapour density of a volatile substance ?

Accepted Answer

The volume of the vapour from a known weight of a volatile liquid at a particular temperature and pressure is known as the vapour density. It is obtained by dividing the molecular weight of the vapour by the average molecular weight of air thus it is unitless, and it is a number.

Determination of the relative molecular mass of volatile liquids by Victor Meyer's method: A known weight of a volatile liquid is vapourised which displaces an equal volume of air. From this volume of collected air and the weight of liquid, the vapour density and then the relative molecular mass of the liquid is calculated.

Victor Meyer's apparatus consists of a long cylindrical glass vessel having a bulb at the bottom, a side tube, and a funnel fitted with a cork at the top.

To start with the experiment the liquid in the outer jacket tube is heated to boiling. Now the side tube of the cylindrical glass vessel is dipped underwater in a trough. When no more bubbles appear. The apparatus has attained a steady temperature. Then a graduated tube filled with water is inverted over the end of the side tube. In the meanwhile, a known weight of the volatile liquid (about 0.1 to 0.2 g) is taken in a small bottle fitted with a loose stopper known as Hofmanns' bottle. This bottle is quickly dropped into the cylindrical glass vessel and the mouth of the funnel of the glass vessel is closed with the cork. Since the Hofmanns' bottle is loosely stoppered, the stopper opens and the volatile liquid vapourises quickly. The vapour so produced displaces the same volume of air that is collected in the graduated tubes at room temperature and pressure. When the water level in the graduated tube remains constant for quite some time, the graduated tube is removed with the thumb at its open end with care so that the water level is constant. Then the graduated tube is carefully lowered in a water jar and the volume of air is noted by making the level of water inside and outside the tube the same. The room temperature and the atmospheric pressure are also noted.

Now since the volume of the vapour from a known weight of a volatile liquid at a particular temperature and pressure is known, the vapour density and the molecular mass of the volatile liquid can be calculated as follows :

$The relative molecular mass = 2 \times V . D . The vapour density of the vapour is given by V . D = \frac{Weight of Vcc of the vapour at NTP}{Weight of the same Vcc of the hydrogen gas at NTP} = \frac{m}{V \times 0.000089} Now the relative molecular mass M = 2.016 \times V . D . = 2 \times V . D .$

Define vapour density. How will you determine the vapour density of a volatile substance ?

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