Question

The reference standard used for defining atomic mass is:
A.) $H - 1$
B.) $C - 12$
C.) $C - 13$
D.) $C - 14$

Answer 1

Hint: The masses of the individual atoms are very-very small such that it makes difficulty in measurements and it increases complexity while measuring in grams therefore, the comparisons of masses is easier when we choose a reference standard of an element from which all other masses could be compared.

Complete step by step answer:
As we know that we use a standard reference for the calculation of atomic masses of all elements. Therefore, scientists have decided the carbon nucleus as the standard reference from which we can compare masses of all other elements. Therefore, one atom of carbon $ - 12$ is assigned a mass of $12\;amu$ where $amu$ is the atomic mass unit. Therefore, the masses of all other elements are compared with atomic mass units.
Here, one atomic mass unit can be defined as the mass which is equal to the mass of one twelfth of an atom of carbon $ - 12$ that is the most common isotope of carbon atom. The value of one atomic mass unit is equal to $1.66 \times {10^{ - 24}}g$. As the reference standard used for defining atomic mass is carbon $ - 12$.

Hence, option B.) is the correct answer.

Note:
Always remember that if an atom has a mass in grams equal to the one-twelfth of the mass of carbon $ - 12$ atom then that element would have a relative mass of $1amu$(one atomic mass unit). As we know, the mass of a carbon atom in grams is $1.99 \times {10^{ - 23}}g$. And mass of hydrogen in grams is $0.1673 \times {10^{ - 23}}g$ which is approx. equal to the one-twelfth of the mass of the carbon atom. Hence, the mass of hydrogen in $amu$ is $1\;amu$.