Question

The ratio of the mass of C-12 atoms to that of an atom of element X (whose atomicity is 4) is 1:9. The molecular mass of the element X is:
a.) 480 $gmo{l^{ - 1}}$
b.) 432 $gmo{l^{ - 1}}$
c.) 36 $gmo{l^{ - 1}}$
d.) 84 $gmo{l^{ - 1}}$

Answer 1

Carbon is a group 14 element. Mass of one atom of carbon is equal to 12 u and the mass of one mole of carbon is equal to 12 u.

Complete Solution :
The atomicity of a molecule is defined as the total number of atoms that constitute a molecule. In the given question, it is given that the atomicity of an unknown element X is equal to 4.
Now,
Atomic mass of carbon-12 atom = 12 u
And the molar mass of carbon-12 atom = 12 g
Given that the ratio of C-12 atomic mass to that of the atomic mass of X = 1:9
$\dfrac{{12}}{{{M_x}}} = \dfrac{1}{9}$
$\therefore {M_x} = 108$
108 grams is the weight of one atom of X.
Given that the atomicity of X = 4
Hence, one molecule will contain 4 X atoms.
So, molar weight of X = 108 x 4
= 432 g.
Hence, the molar weight of X = 432 g
So, the correct answer is “Option A”.

Note: Note that molecular mass and molar mass are two different quantities where molar mass is the mass of one mole of a substance and molecular mass is the mass of a molecule of the substance and this difference is not to be confused.