Question

Assertion: One atomic mass unit is defined as one-twelfth of the mass of one carbon-12 atom.
Reason: Carbon-12 isotope is the most abundant isotope of carbon and has been chosen as the standard.
A.Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
B.Both Assertion and Reason true but Reason is not the correct explanation of Assertion.
C.The assertion is true but Reason is false.
D.Both Assertion and Reason are false.

Answer 1

To answer this question, you should recall the basic concepts of atomic mass and the definition of the atomic mass unit or amu. The carbon atom had been used as a reference while assigning atomic mass to the elements of the periodic table.

Complete step by step answer:
According to the standard definition of the atomic unit of mass, it is defined as accurately $\dfrac{1}{{12}}$ of the mass of a carbon-12 atom. Hence, $1{\text{ amu}}$= Average of the proton rest mass and the neutron rest mass.
1 amu =$1.67377{\text{ }} \times {\text{ }}10{\;^{ - 27}}{\text{ kg = }}1.67377{\text{ }} \times {\text{ }}10{\;^{ - 24}}\;{\text{g}}$
Using mass spectrometry, the mass of one atom can be measured relative to the mass of another.
Hence, both the statements are correct but Reason is not the correct explanation of Assertion.

So the correct answer is option B

Note: The concept that a mole of any substance contains the same number of particles was formed out of research which was conducted by Italian physicist Amedeo Avogadro. Avogadro constant can be defined as the number of molecules, atoms, or ions in one mole of a substance: $6.022 \times {10^{23}}$ per mol. It is derived from the number of atoms of the pure isotope $^{{\text{12}}}{\text{C}}$in 12 grams of that substance and is the reciprocal of atomic mass in grams. Now the mole concept can be applied to ions and formula units.
a)1 mole of ${O_2}$means Avogadro's number of oxygen molecules and it will be equal to 2 times Avogadro's number of oxygen atoms.
b)1 g-molecule of ${O_2}$ is the same as one mole of the oxygen molecule and contains Avogadro's number of nitrogen molecules and twice this number of atoms.
c)1 g-atom of Nitrogen means 1 mole of nitrogen atoms and contains Avogadro's number of nitrogen atoms. The mass of 1 mole of any species is equal to its molar mass.
The formulae for the mole concept can be summarized as:
${\text{No}}{\text{. of moles = }}\dfrac{{{\text{Mass of the Substance in grams}}}}{{{\text{Molar mass of a Substance}}}} = \dfrac{{{\text{Number of Atoms or Molecules}}}}{{6.022 \times {{10}^{23}}}}$