Question

If the atomic mass unit ‘u’ were defined to be ${{\dfrac{1}{5}}^{th}}$ of the mass of an atom C-12, what would be the atomic weight of nitrogen in amu or ‘u’ in this state? Atomic weight of N on conventional scale is 14:
(A) 6.77 u
(B) 5.834 u
(C) 14 u
(D) 23 u

Answer 1

Hint: An atom is composed of two regions. One region is in the center of the atom containing protons and neutrons are called the nucleus and the other which is the outer region holds electrons in orbit around the nucleus. Protons and neutrons have approximately the same mass as the atomic mass unit (amu).

Complete answer:
1 atomic mass unit (amu) = $1.67\times {{10}^{-24}}gms$
Generally, atomic mass is defined as the number of protons or electrons of the atom. Since many elements have isotopes, they use average atomic mass. In the periodic table, the mass of carbon was reported as 12.011amu, which is the average mass of the carbon atom.
The atomic mass on the relative scale is C – 12 and all the masses of the elements are determined relative to C – 12.
Relative atomic mass = $\dfrac{mass~of~an~atom}{\dfrac{1}{12}of~C-12}$
Now consider ${{\dfrac{1}{5}}^{th}}$ of the mass of an atom C-12, then the relative atom mass be
Relative atomic mass = $\dfrac{mass~of~an~atom}{\dfrac{1}{5}of~C-12}$
Weight of N on conventional scale = 14 based on ${{\dfrac{1}{12}}^{th}}$ a standard scale,
Now according to the given scale the atomic mass unit ‘u’ was defined to be ${{\dfrac{1}{5}}^{th}}$ of the mass of an atom C-12 = $14\times \dfrac{12}{5}=5.834u$

Hence, the correct answer is option B.

Note:
The standard unit of the mass of an atom is the atomic mass unit, which is equal to ${{\dfrac{1}{12}}^{th}}$the mass of an atom C-12. The atomic mass unit is also called as Avogram, which is a unit mass and weight equal to one gram divided by Avogadro’s number.