Question

In the following questions, two statements are given, one is Assertion (A) and the other is Reason (R). Examine the statements carefully and mark the correct answer according to the instructions given below:
Assertion: The standard unit for expressing atomic mass is amu.
Reason: Nowadays 'amu' is represented by ‘u’.

A. If both A and R are correct and R is the correct explanation of A
B. If both A and R are correct and R is not the correct explanation of A
C. If A is correct R is wrong
D. If both A and R are false

Answer 1

Hint: The atomic mass is the mass of an atom that is very small as atoms are exceptionally small. However, the current system of atomic masses is established taking carbon-12 as the standard.

Complete Step by Step Solution:
Carbon-12 is one of the isotopes of carbon and can be depicted as ¹²C.
In this procedure, carbon-12 is assigned a mass of precisely 12 atomic mass units (amu) and masses of all different atoms are given by comparing to this standard.
One atomic mass unit is defined as a mass the same as one-twelfth the mass of one carbon-12 atom.

1 amu = \[{\rm{1}}{\rm{.663 \times 1}}{{\rm{0}}^{{\rm{ - 24}}}}\]g
Mass of an atom of H = \[{\rm{1}}{\rm{.673 \times 1}}{{\rm{0}}^{{\rm{ - 24}}}}\]g
So, in terms of amu, the mass
= ($\dfrac{1}{1.663\times 10^{-24}}\times 1.673\times 10^{-24}$)
= 1.00 amu

Mass of an atom of O = \[{\rm{2}}{\rm{.6561 \times 1}}{{\rm{0}}^{{\rm{ - 23}}}}\]g
Mass of an atom of O = $\dfrac{2.6561\times 10^{-23}}{1.673\times 10^{-23}}$
= 15.99 amu
= 16 amu (approximately).

Currently, there is a replacement of ‘amu’ with ‘u’ which is called a unified mass.

Hence, the assertion and the reason are correct. But the reason is not the correct explanation of the assertion.

So, option B is correct.

Note: The 'amu' was used for expressing atomic mass as the size of an atom is very small. So, the mass of an atom is also very little. A system was set by taking carbon-12 isotope as the standard mass. The mass of all other elements was calculated based on the mass of the carbon-12 isotope.