Question

Modern atomic mass scale is based on the mass of:
$A.\;H - 1$
$B.\;C - 12$
$C.\;C - 14$
$D.\;C - 16$

Answer 1

Hint: The atomic mass refers to a given atom's actual physical mass. The atomic mass in the numerical values is very small which is why it is measured on the basis of comparison with the carbon-12 isotope mass.

Complete step-by-step answer:
The atomic mass of an atom is the mass of an atom. The SI unit of mass is kilogram , the atomic mass is also represented in the non-SI unit Dalton, where 1 Dalton is defined as $\dfrac{1}{{12}}$ of the mass of a single carbon-12 atom, at rest. Nucleus which consists of protons and neutrons accounts for almost all of the total mass of atoms, with the electrons and nuclear binding energy making minor contributions. Thus, when expressed in Daltons, the numerical value of the atomic mass is almost the same as the mass number. Also one atomic mass unit is equal to ${1.6605389210^{ - 24}}g$ or ${1.6605389210^{ - 27}}kg$ which is equal to $\dfrac{1}{{12}}$of the mass of an atom of carbon-12.
It is expressed as ${m_u} = \dfrac{{m({}^{12}C)}}{{12}} = 1\;Da$, where ${m_u}$is the molar mass constant. And $m({}^{12}C)$ is the molar mass of Carbon-12.

Hence option B is the correct answer and the atomic mass scale is based on $C - 12$ also known as Carbon-12.

Additional Information:
Carbon-12 (12C) is the most abundant of the two stable carbon isotopes (the other being carbon-13), containing 98.93 percent of the carbon element. Its abundance is due to the triple-alpha cycle by which it is produced in stars. Carbon12 is of special interest in its use as the norm by which atomic masses of all nuclides are determined and its atomic mass thus by definition is exactly 12 daltons. Carbon-12 comprises six protons, six neutrons, and six electrons.

Note: For answering this type of question we must remember the definition of atomic mass unit and its value. Also one atomic mass unit is equal to ${1.6605389210^{ - 24}}g$ or ${1.6605389210^{ - 27}} kg$ which is equal to $\dfrac{1}{{12}}$ of the mass of an atom of carbon-12.