Determining Molecular Masses Using Victor Meyer’s Method

JEE CRASH  Lite COURSE 2022

Introduction to Victor Meyer’s Method

It is counted as a standard laboratory method for determining the molecular weight of a volatile liquid. This concept was discovered by Victor Meyer. In this process, a known mass of volatile liquid or solid is put under examination and they are converted into its vapour form by heating by using Victor Meyers tube, as this tube helps in the conversion of volatile solid or liquid into vapour form. Vapour helps in the displacement of its own volume of air and the volume of air displaced at a given experimental temperature and pressure is calculated. At STP ( standard temperature pressure ) i.e at  \[ 2.24 \times 10^{-2}m^{3}\] displaced mass of air is calculated. The value which is obtained from this process is known as the molecular mass of the substance.

 

The Objective of Determining Molecular Masses using Victor Meyer’s Method

To find out the relative molecular mass of unknown liquid mistreatment of the Victor Meyer equipment.


The Victor Meyer technique contains vaporising a well-known weight of liquid in a during an exceedingly in a chamber maintained at an acceptable and constant extreme temperature. The air displaced from the chamber by the vaporised sample is cooled to temperature, and its volume is rigorously measured. Substitution of air for the particular vapour, therefore, provides a way of deciding the amount the well-known mass of vapour would occupy at temperature if it might be cooled while not condensing.

 

Construction

Instrumentation of this process includes an inner Victor Meyer's tube, the lower end of which is in the form of a bulb. The upper end of the tube has a side tube that leads to a trough filled with water. The tube which is known to be Victor Meyer’s tube is surrounded by a jacket from outside and a liquid is placed in an outer jacket, which usually boils at least 30k more than the substance when kept under examination. Usage of Meyers tube is used to protect the instrument from outer breakage when a glass bottle containing the substance under examination is dropped to it.


An aspherical bottom flask full of a liquid whose B.P. is 10c  over the volatile liquid acts as an associate degree outer glass jacket. A victor Mayor’s tube with an associate degree outer tube acts as an inner glass jacket. This outer tube is lordotic in a very trough fitted with water. The very cheap Victor Mayor’s tube consists of Hg or amphibole items for padding.


The outer glass jacket is heated thanks to the that air expands and bubbles through the water within the trough. A tiny low glass tube is thought of as Hoffman’s bottle with a stopper that is clean, washed and dried, and weighted. The volatile liquid is taken in Hoffman’s bottle and weighted. A measuring instrument tube full of water is placed over the tube connected to victor civil authority equipment.


Now Hoffman’s bottle is born within the Victor civil authority tube. Thanks to heat, liquid within the bottle evaporates and blows off the stopper and displaces air that corresponds to its volume that is collected in a very measuring instrument tube by downward displacement of water.


The water temperature and pressure are noted. The measuring instrument tube is taken into another trough full of water for equalisation of pressure. The quantity of air displaced is noted, which corresponds to the quantity of the vapours.

 

Numerical Representation of Victor Meyer Method

This method is used for the determination of the molecular mass of volatile organic compounds. Under this process, we take the known mass of the compound, and this compound is vaporised in an instrument known as Victor Meyer tube.  The amount of vapour obtained is measured and reduced to standard temperature pressure i.e STP.

Let the volume of vapours obtained at STP be V mL

22400 mL of vapours are obtained from 1 mole of a given compound.

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

Mole= W/Mw

Therefore,

W/Mw = V/22400


Question 1

Determination of Victor Meyer’s the following were made:

Mass of compound = 0.17g

Collection of volume air  = 34.2 mL

Temperature = 15 c

Atmospheric Pressure = 750 mm

Vapour pressure of water at 15 c = 13 mm

Calculate the vapour density and the molecular mass of the compound.

Solution:

Given \[ V_{1} \]= 34.2 ml

\[ V_{2} \]=?

\[ P_{1} \] = ( 750-13) = 737 mmm

\[ P_{2} \] = 760mm

\[ T_{1} \] = (15+273) = 288K

\[ T_{2} \] = 273K

By gas equation we get,

\[V_{2} = \frac{737 \times 34.2 \times 273}{288 \times 760}\]

= 31.43767 mL

Vapour density =   W/ V2 x 0.00009

= 0.17/ 31.4376 x 0.00009

= 60.08

Molecular Mass = 2 x vapour density

= 2 x 60.08

= 120.16

 

Applications of Victor Meyer Method

  1. Identification of Alcohols

This method is used for the identification of alcohol. From this, you can determine primary, secondary, and tertiary alcohols by a change in their colours. During this process, alcohol is treated with Phosphorus triiodide which is further treated with Silver nitric oxide to get nitroalkane as a product. Further, this product i.e nitroalkane is treated with nitrous acid which is obtained by reaction of NaNO2 and HCl. The last wrestling solution is treated with KOH and the required colour is obtained. If it is red the colour indicates primary alcohol, if blue it indicates secondary alcohol and if no colour is seen then indicates tertiary alcohol.


Alcohols are the hydroxyl derivatives of alkanes which contains - OH functional group. These compounds are usually obtained from the alkanes after replacing the hydrogen atoms with a hydroxyl group. An alcohol that contains two-OH groups is called Dihydric alcohol.


  1. Determination of the Empirical Formula and the Molecular Formula

This method is used for the determination of both empirical as well as molecular formulas, by using the given method. An empirical formula tells us the relative ratios of different atoms in a compound.


Whereas to find a molecular formula, we need to know the molecular mass of the given compound.

 

What is Molecular Formula?

Molecular formula is derived from the molecules in an atom. It represents the total number of individual atoms present in a molecule.


A molecular formula also has a subscript. This subscript reports the actual number of each type of atom in a molecule of the compound.

  • The Molecular formula is also associated with gram molecular masses. These are simple whole Numbers of the corresponding empirical formula mass.

 

What is an Empirical Formula?

  • The empirical formula is the simplest formula for a compound. The empirical formula is defined as the ratio of subscripts of the smallest possible whole number of the elements present in the formula. It is generally the simplest known formula for a compound.

  • An empirical formula for a compound is the formula of a compound written with the smallest integer subscript.

  • The empirical formula provides information about the ratio of numbers of atoms in a compound. The percent composition of a compound directly leads to its empirical formula.

The relation between the molecular formula and the empirical formula is

Molecular Formula =  n x Empirical formula

 

Empirical Formula

An empirical formula represents the simplest whole-number ratio of various atoms present in a compound.

Example:  the empirical formula of acetylene is CH

 

Molecular Formula

The molecular formula is the representation of the exact number of different types of atoms present in a molecule of a compound.

Example: For Acetylene the empirical formula is \[C_{2}H_{2}\]

FAQs on Determining Molecular Masses Using Victor Meyer’s Method

1. Differentiate between primary, secondary, and tertiary alcohol by Victor Meyer Method?

The Victor Meyer test is used to differentiate between primary, secondary and tertiary alcohol by the colour they obtain when they react with the required reagents.

  • Primary Alcohols- Red colour

  • Secondary Alcohols- Blue colour

  • Tertiary Alcohols- No colour is observed

2. Write a note on the instrumentation of the Victor Meyer Method?

Instrumentation of this process includes an inner Victor Meyer's tube, the lower end of which is in the form of a bulb. The upper end of the tube has a side tube that leads to a trough filled with water. The tube which is known to be Victor Meyer’s tube is surrounded by a jacket from outside and a liquid is placed in an outer jacket, which usually boils at least 30k more than the substance when kept under examination. Usage of Meyers tube to protect the instrument from outer breakage,  when a glass bottle containing the substance under examination is dropped to it.

3. How to make an empirical table of carbon’s composition is 48%, hydrogen 8%, nitrogen 28%, and oxygen 16%?

The required empirical table is drawn below:

Element

Percentage

Atomic Mass

The relative number of Atoms

Simplest Atomic Ratio

Simplest whole number Atomic Ratio

Carbon

48.0

12

48/12=4

4/1=4

4

Hydrogen

8.0

1

8/1=8

8/1=8

8

Nitrogen

28.0

14

28/14=2

2/1=2

2

Oxygen

16.0

16

16/16=1

1/1=1

1


So the obtained empirical formula is \[C_{4}H_{8}N_{2}O\].

4. Mercury forms a compound with chlorine that is 73.9% mercury and 26.1% chlorine by mass. What is the empirical formula?

Mercury dichloride

5. Which colour is obtained in secondary alcohol during the alcohol detection?

Blue colour

6. What are the merits of Vector Mayer’s method?

The method is very simple to carry out the weight.

The illustration required for the experiment is very small.

7. What are the demerits of Vector Mayer’s Method?

This method applies to volatile liquid only.

The procedure can not be used for the substance which undergoes thermal decomposition.

8. How do you determine molecular weight by Victor Meyer's method?

Victor Mayer tube is used to vaporize a known mass of the compound. The vapors are applied from the sample and it displaces an equal number of filled air from it into a graduated tube. The measured volume of vapors is reduced to STP.

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