Question

The total percentage of magnesium present in magnesium nitrate crystals $\left[ Mg{{(N{{O}_{3}})}_{2}}\cdot 6{{H}_{2}}O \right]$ is $[Mg=24;N=14;O=16;H=1]$
(A) $9.38$
(B) $18.76$
(C) $11.93$
(D) $27.43$

Answer 1

We know that the relative mass of an object is the mass of the object compared to that of a standard object. Mass number and abundance of isotopes tell about the relative atomic masses. Atomic mass is different from atomic number.

Complete answer:
Atom has a nucleus containing protons, neutrons and electrons revolving around the shell. Mass of the atom is negligible. Therefore relative atomic mass can be used. . Relative atomic mass is the average mass of one atom when compared with $\dfrac{1}{2}th$ of mass of carbon-12 atom. Carbon is taken as a standard because only a carbon atom has exactly a whole number as its mass. Relative molecular mass of a molecule is the sum of relative atomic masses of all the atoms in a molecule. Relative formula mass of a molecule is the sum of relative atomic masses of all atoms in the formula. Chemical formula of magnesium nitrate is given as $\left[ Mg{{(N{{O}_{3}})}_{2}}\cdot 6{{H}_{2}}O \right]$
Generally atomic mass is the number of protons and neutrons in an atom. Calculation of relative formula mass: First calculate the number of each element in the molecule with the given formula. Add the atomic masses of each element
Molar mass of $\left[ Mg{{(N{{O}_{3}})}_{2}}\cdot 6{{H}_{2}}O \right]=256gm.$
So, the percentage of magnesium is given by $=24\times \dfrac{100}{256.4}=9.38$ here $24$ is atomic mass of magnesium.
Therefore, the correct answer is option A.

Note:
Remember that generally atomic mass is the weight of an atom. $\dfrac{1}{2}th$ of mass of the $carbon-12$ atom is termed as (atomic unit mass). Mass of one carbon atom is $12amu.$ Relative formula masses of ionic compounds can also be calculated using this method.