Question

Mass of an electron in amu is _______.
a.) 0.00055
b.) 0.0055
c.) 0.000055
d.) 0.055

Answer 1

Hint: Mass of an electron in amu unit i.e. atomic mass unit can be found by dividing the mass of an electron in kilograms to mass of one atomic unit. The one atomic mass unit is related to the mass of a carbon-12 atom.

Complete step by step solution:
Let us find the value of mass of an electron step by step. As we know, the mass of an electron in kilograms is 9.1$\times$ 10$^{-31}$kg.
Now, we will define the atomic mass unit. It is equal to $\dfrac{1}{12}$th of the mass of a carbon-12 atom, or we can say it is the mass of a nucleon, whether it is a proton, or a neutron.

As we know, Avogadro’s number is equal to 6.022$\times$ 10$^{-23}$atoms in 1 mole, then for carbon-12 atom, it is 1.99 $\times$ 10$^{-23}$ g/atom.
Thus, 1 atomic unit is equal to 1.66 $\times$ 10$^{-27}$ kg.
Therefore, as mentioned, mass of an electron in amu = $\dfrac{9.1\times 10^{-31}}{1.66\times 10^{-27}}$ = 0.00055 u (approximately).

Thus, the mass of an electron in amu is 0.00055 u. The correct option is (A).

Note: Don’t get confused with the calculation of the value of one atomic mass unit in kilograms. We know that a carbon -12 atom consists of six protons, and six neutrons with the molar mass of 12 g/ mol. So, we found the value by using Avogadro's number for 12g/ mol. The value attained in g/atom is converted into kg, as we required in the kilogram unit.