Question

Assertion -The atomic mass of potassium is $39$ .
Reason – An atom of potassium is $39$ times heavier than $\dfrac{1}{{12}}$ th of the mass of carbon atom $({C^{12}})$ .
Read the above assertion and reason and choose the correct option regarding it.
A . Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B . Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C . Assertion is correct but Reason is incorrect.
D . Assertion is incorrect but Reason is correct.

Answer 1

Hint : We know that the atomic mass of potassium is $39$,we can verify this from a periodic table. The sum of protons and neutrons is equal to atomic mass. In potassium atoms, there are $19$ protons (atomic number) and $20$neutrons so the atomic weight of potassium is the sum of the number of protons and neutrons equals $39$.

Complete answer:

Atomic mass of any is expressed in amu (atomic mass unit),which is equal to $\dfrac{1}{{12}}$ th of the mass of carbon atom $({C^{12}})$ equal to $1.6605 \times {10^{ - 24}}$ g. Atomic mass of potassium is $39$ times heavier than $\dfrac{1}{{12}}$ th of the mass of carbon-12. One an amu is $\dfrac{1}{{12}}$ of the mass of on carbon -12 atom then it is equal to $\dfrac{1}{{{N_A}}}$; where ${N_A}$ is Avogadro’s number so $\dfrac{1}{{{N_A}}} = \dfrac{1}{{6.022 \times {{10}^{23}}}}$ $= 1.66 \times {10^{ - 24}}$ g.

Additional information : Atomic mass unit is defined as $\dfrac{1}{{12}}$ of the mass of an unbounded neutral carbon-12 in its ground state at rest. Dalton or unified atomic mass unit is a unit of mass, represented by $Da$ and $u$ respectively. This unit is used to express mass of atoms, molecules and elementary particles. Hydrogen has a relative mass of one amu equal to $1.66 \times {10^{ - 27}}$ kilograms.
Carbon-12 is a solid which used to be chemically stable and safe to use and handle. Hence both assertion and reason are correct and reason is the correct explanation for assertion so option A is correct .

Note : We have approached this problem with the help of the concept of amu and we also remember the atomic mass of potassium as it is a very useful element of the periodic table. In assertion reason problems we should firstly check the assertion and reason statement then try to find out the explanation of the problem.