Question

How do you calculate the average atomic mass of magnesium, given the following percent abundances and isotopic masses: 78.99 % $${}^{24}Mg$$ (23.98504u), 10.00 % $${}^{25}Mg$$ (24.98584u) and 11.01 % $${}^{20}Mg$$ (25.98259u)?

Answer 1

Hint: In order to calculate the average atomic mass of magnesium, we must first know what the average atomic mass is. We have to also see that the percentages of all the three given isotopes of Magnesium is equal to 100.

Complete step by step answer:
Let us first understand what an atomic mass is. Atomic mass is said to be the mass of an atom. The atomic mass is usually expressed in u. The masses of the proton, neutrons and the electrons together will give the atomic mass of the atom. We know that the mass of the electron is very much lesser compared to the mass of the proton and neutrons. Therefore, the masses of the electron will not influence that much in the calculation of atomic mass of an atom.
- Let us now move on to the given question.
The isotopes of magnesium with the percentage abundance can be written as:
$${}^{24}Mg$$ = 78.99 % of 23.98504 u
$${}^{25}Mg$$ = 10.00 % of 24.98584 u
$${}^{20}Mg$$ = 11.01 % of 25.98259 u.
$$\text{average atomic mass = }{\displaystyle \frac{(78.99\times 23.98504)+(10.00\times 24.98584)+(11.01\times 25.98259)}{100}}=24.30505u$$
Therefore, the average atomic mass of magnesium is found to be 24.30505 u.

Note: - We have to remember that atomic mass and the molar mass are not the same.
- Atomic mass is applicable only for the atoms.
- But molar mass is applicable to anything that can be present in moles.