Question

In Victor mayer's method, 0.2 g of an organic substance displaced 56 mL of air at STP, the molecular weight of the compound is:
(A) 56
(B) 112
(C) 80
(D) 28

Answer 1

Hint: Understand the physical conditions prevalent at STP i.e. standard temperature and pressure. It is important to understand the working of the victor meyer test and the reaction mechanism. The mass of the compound that will displace 22.4 L at STP will be the molecular or molar mass of the organic compound.

Complete Solution :
The Victor Mayer method is used for mainly organic compounds. A known mass of the compound is vaporized in the apparatus called Victor Mayer tube.
The vapours thus obtained, displace an equal volume of air into a tube that is graduated. The volume that is measured is reduced to the standard temperature and pressure conditions.
This is done to ease our calculation process.

Let the volume of vapours at STP be V mL.
We know that 1 mole of a compound will occupy 22.4 L or 22400 mL of volume at STP.
Mole = $\dfrac{\text{Weight(W)}}{\text{Molecular weight(MW)}}$
Equating this to volume, we get
$\dfrac{\text{V}}{22400}$ = $\dfrac{\text{Weight(W)}}{\text{Molecular weight(MW)}}$
In this way we will now determine the molecular mass of the organic compound given above.
W = 0.2 g
V = 56 mL
Substituting these values, we get
$\dfrac{56}{22400}$ = $\dfrac{0.2}{\text{MW}}$
Molecular weight = $\dfrac{0.2\text{ x 22400}}{56}\text{ = 80 g}$
The molecular weight of the organic compound is calculated as 80 g.
So, the correct answer is “Option C”.

Note: Victor Mayer method is used for mainly organic compounds. A known mass of the compound is vaporized in the apparatus called Victor Mayer tube.
The vapours thus obtained, displace an equal volume of air into a tube that is graduated. The volume that is measured is reduced to the standard temperature and pressure conditions.
This is done to ease our calculation process.

Let the volume of vapours at STP be V mL.
We know that 1 mole of a compound will occupy 22.4 L or 22400 mL of volume at STP.
Mole = $\dfrac{\text{Weight(W)}}{\text{Molecular weight(MW)}}$
Equating this to volume, we get
$\dfrac{\text{V}}{22400}$ = $\dfrac{\text{Weight(W)}}{\text{Molecular weight(MW)}}$
In this way we will now determine the molecular mass of the organic compound given above.
W = 0.2 g
V = 56 mL
Substituting these values, we get
$\dfrac{56}{22400}$ = $\dfrac{0.2}{\text{MW}}$
Molecular weight = $\dfrac{0.2\text{ x 22400}}{56}\text{ = 80 g}$
The molecular weight of the organic compound is calculated as 80 g.