Determining Molecular Masses Using Victor Meyer's Method

What Is Victor Meyer’s Method?

Victor Meyer’s method or test is used to measure the vapour density of volatile organic compounds. A known mass of the compound is vaporised in an instrument called a Victor Meyer tube. Vapours are obtained from the sample, and it displaces an equal volume of air into a graduated tube. The volume of vapours is then measured and reduced to STP.

Let the volume of vapours at STP be V mL

22400 mL of vapours are obtained from 1 mole of the compound.

V mL of vapours are obtained from (V/ 22400) mL of the compound

Mole = W/Mw

Therefore,

W/Mw = V/22400

From this, the molecular weight of the compound can be determined.

Also Read

• Organic Chemistry
• Inductive Effect
• Purification
• Qualitative Analysis
• Lassaigne’s test

Determination of Empirical and Molecular Formula

If the percentage compositions of the elements are given, then use the following method: For example, if carbon’s composition is 48%, hydrogen 8%, nitrogen 28% and oxygen 16%, make a table with the given data as follows:

To calculate the relative number of atoms, divide the percentage by the atomic mass of the element.

To find the simplest atomic ratio, divide the relative number of atoms, and with the lowest number that is obtained, in this particular case, among 4, 8, 2 and 1, it is 1.

To find the simplest whole number, multiply each simplest atomic ratio with an integer to make it a whole number. In this particular case, we can skip it since all the numbers are whole numbers.

Therefore, the empirical formula of the compound is C₄H₈N₂O.

To find out the molecular formula, we need the molecular mass of the compound.

Say, for the above question, the molecular mass of the substance had been given as 200 amu; we need to find out the empirical mass.

Empirical mass = 4×12+8×1+2×14+1×16

= 100 amu

The logic here is that we have to find out by how many times the molecular mass is more than the empirical mass. So, we divide them.

n = (Molecular mass)/(Empirical mass)

n = 200/100

n = 2

Now, we found that the molecular mass is twice, so the molecular formula should be = Empirical formula x 2

Molecular formula= C₄H₈N₂O x 2 = C₈H₁₆N₄O₂