Question

What is the mass of one atom of Ca?
40 amu or $\dfrac{{40g}}{{{N_A}}}$

Answer 1

Ca is a chemical symbol of atom calcium. Recall the concept of atomic mass unit. One atomic mass unit is defined as a mass exactly equal to one-twelfth the mass of one carbon-12 atom. $^{12}C$ is assigned a mass of exactly 12 atomic mass units and masses of all other atoms are given relative to this standard.

Complete answer:
The atomic mass or the mass of one atom is actually very-very small quantity. But in the nineteenth century, scientists could determine the mass of an atom relative to another by experimental means. The present system of atomic masses is based on carbon-12 as the standard. In this system, $^{12}C$ is assigned a mass of exactly 12 atomic mass units (amu) and masses of all other atoms are given relative to this standard.
One atomic mass unit (amu) is defined as a mass exactly equal to one-twelfth the mass of one carbon-12 atom. Mass of one hydrogen atom is 1 amu.
Similarly, the mass of one atom of calcium ($Ca$) is thereby calculated as 40 amu. Therefore, 40 amu is the correct option.
Hence, this is the required answer.

Note:
The mass of one mole of a substance in grams is called its molar mass/ atomic mass/ in amu. One mole is the amount of a substance containing as many particles or entities as there are in atoms in exactly $^{12}C$ isotope. Number of entities in 1 mol is given by the name ‘Avogadro constant’, denoted by ${N_A}$ and has value $6.022 \times {10^{23}}$ atoms/mol.
Thus, we can say that 1 mole of Ca atoms = $6.022 \times {10^{23}}$ (${N_A}$) atoms.
But, 1 mole of Ca = 40 amu.
Therefore, 40 amu = $6.022 \times {10^{23}}$ atoms or ${N_A}$ .