Question

The value of 1 amu is equal to:
(A) 1.66 $\times$ ${{10}^{-8}}$g
(B) 1.66 $\times$ ${{10}^{-4}}$g
(C) 1.66 $\times$ ${{10}^{-16}}$g
(D) 1.66 $\times$ ${{10}^{-24}}$g

Answer 1

The full form of amu is atomic mass unit. To answer this question we should be very clear with the definition of atomic mass unit. This may help us to remember the concept and value forever.

Complete answer:
The atomic mass unit is defined as $\dfrac{1}{12}$ of the mass of the ${}^{12}C$isotope. The carbon mentioned here has six protons and six neutrons in its nucleus. The sum of masses of the proton and neutron is approximately equal to 1amu.
According to the definition let's find the value of 1amu:
1 amu = $\dfrac{1}{12}$ $\times$mass of carbon atom (${}^{12}C$isotope)
Atomic mass of carbon atom = 12g
Let's know the definition of Avogadro number which may help in calculating the value of 1 amu
Avogadro number is the number of atoms in 12g of carbon- 12 isotope. Thus the Avogadro number is 6.022 $\times$ ${{10}^{24}}$.
We know that,
6.022 $\times$ ${{10}^{24}}$of carbon atom = atomic mass in gram (12g)
1 amu = $\dfrac{1}{12}$ $\times$mass of carbon atom (${}^{12}C$isotope) = 6.022 $\times$ ${{10}^{24}}$
So, $\dfrac{12}{6.022\times {{10}^{24}}}$
So, 1 amu = $\dfrac{1}{12}$ $\times$ $\dfrac{12}{6.022\times {{10}^{24}}}$
= 1.66 $\times$ ${{10}^{-24}}$g
The relative mass of atoms, the elements of different isotopes are expressed in amu.
Thus, from the above discussion we can conclude that option D is the correct answer.

Note:
It is very important to understand the exact concept behind each definition which may help us in solving questions like this within no time and this helps us remember the definition forever. One amu is also called one Dalton. Atomic weight is also expressed in terms of atomic mass unit.