Question

Vapor density of metal chloride is 77. Equivalent weight of metal is 3 then, its atomic weight will be:
A. 3
B. 6
C. 9
D. 12

Answer 1

Equivalent weight which is also known by the name gram equivalent is defined as the mass of one equivalent which can be simplify in a manner that mass of a given substance which will combine with or we can also explain it which will displace a fixed quantity of another substance.

Complete answer:
The equivalent weight of an element can be defined as the mass which combines with or displaces 1.008 gram of hydrogen or 8.0 grams of oxygen or 35.5 grams of chlorine. These values correspond to the atomic weight which is divided by its valency.
Atomic weight can be calculated by the product of equivalent weight and its valency where valency can be calculated by the formula:
$\dfrac{2\times vapor\ \text{density}}{Equivalent\ \text{weight of metal+35}\text{.5}}$
Vapor density = 77 (Given), Equivalent weight of metal = 3 (Given), put the values in the equation:
$\dfrac{2\times 77}{3\text{+35}\text{.5}}=\dfrac{154}{38.5}=4$
Now as above discussed atomic weight will be equal to the product of equivalent weight and its valency
Then atomic weight = $4\times 3=12$

Hence option D is the correct answer.

Note:
Atomic weight can also be known by relative atomic mass which means ratio of the average mass of a chemical element atoms to some standard value. The standard unit of atomic mass is given by one-twelfth the mass of an atom of the isotope carbon-12. It is measured in terms of atomic mass units denoted by amu which is also called daltons.