Question

\[3.011 \times {10^{22}}\] atoms of an element weigh 1.15 gm. The atomic mass of the element is:
(A) 23
(B) 10
(C) 16
(D) 35.5

Answer 1

Hint: In order to find the atomic weight of an element, we must first know what an atomic weight is. Atomic weight can also be referred to as the relative atomic mass. Atomic weight or mass is said to be the average mass of atoms present in an element.

Complete Solution :
- Let us first understand about the atomic weight. Atomic weight or mass is said to be the average mass of atoms present in an element. Atomic weight is measured in the atomic mass units (i.e., amu). Atomic mass is the total number of neutrons and protons present in an atom along with some extra added by the electron. The electron is having a very low mass compared to the mass of neutrons and the protons. Therefore, the mass of an electron is not that much influenced in the calculation of atomic mass of an atom.
- To find the atomic mass of an element we have to convert the number of atoms to the moles.
\[{\rm{Atomic~mass = }}\dfrac{{1.15}}{{0.05}} = 23\]
Number of atoms = \[3.011 \times {10^{22}}\]
Avogadro number = \[6.022 \times {10^{23}}\]
\[{\rm{Number~of~Moles = }}\dfrac{{3.011 \times {{10}^{22}}}}{{6.022 \times {{10}^{23}}}} = 0.05\]
Therefore,
\[{\rm{Atomic~mass = }}\dfrac{{{\rm{Given~mass}}}}{{{\rm{no}}{\rm{. ~of~moles}}}}\]
\[{\rm{Atomic~mass = }}\dfrac{{1.15}}{{0.05}} = 23\]
So, the correct answer is “Option A”.

Note: We have to remember that atomic mass is different from the mass number and atomic number. Atomic number is the total number of protons or electrons present in an atom, which is represented by the symbol Z. Mass number is the sum of the total number of protons and neutrons present in an atom, which is represented by the symbol A.