Question

Chlorophyll, the green coloring matter of plants, contains 2.68% of magnesium by mass. Calculate the number of magnesium atoms in 3.00 g of chlorophyll. (Atomic mass of magnesium = 24.3 g)

Answer 1

Atomic mass number is given by the total number of protons and neutrons. These have the same number of protons but different numbers of neutrons and the number next to an isotope is mass number. The mass number reports the mass of the atoms present in the nucleus in atomic mass units represented by the term amu.

Complete answer:
To calculate the number of magnesium atoms in 3.00 g of chlorophyll we first have to know about the Avogadro number where Avogadro number can be defined by the concept of mole where a mole of a substance or particle can be defined as containing exactly $6.02214076\times {{10}^{23}}$ particles which may be atoms, molecules or ions where $1.98\times {{10}^{21}}$is known as the Avogadro’s number.
Mass of magnesium in 3.00 g of chlorophyll will be equal to $\dfrac{2.68\times 3}{100}=0.080g$
And we can say that 24.3 g of magnesium contain $6.02\times {{10}^{23}}$atoms
Then 0.08 g of magnesium will contain $\dfrac{6.02\times {{10}^{23}}}{24.3}\times 0.080$
This will be equal to $1.98\times {{10}^{21}}$atoms.
Hence we can say that the number of magnesium atoms in 3.00 g of chlorophyll is $1.98\times {{10}^{21}}$.

Note:
Atomic number and mass number are always whole numbers because these are calculated by counting whole objects i.e. protons, neutrons, and electrons. The sum of the mass number and the atomic number for an atom corresponds to the total number of subatomic particles present in the atom.