Question

The compound magnesium nitrate has the formula ${\text{Mg}}{\left( {{\text{N}}{{\text{O}}_3}} \right)_2}$. What is the relative formula mass of magnesium nitrate?

Answer 1

Hint: 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 step by step solution:
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 $\dfrac{1}{{12}}$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 ${\text{Mg}}{\left( {{\text{N}}{{\text{O}}_3}} \right)_2}$. It has one ${\text{Mg}}$, two ${\text{N}}$and six ${\text{O}}$.
Atomic mass (${{\text{A}}_{\text{r}}}$) of magnesium$= 24$
Atomic mass of nitrogen $= 12$
Atomic mass of oxygen $= 16$
Atomic mass of total ${\text{Mg}}$ in the formula $= 1 \times 24 = 24$
Atomic mass of total ${\text{N}}$ in the formula $= 2 \times 14 = 28$
Atomic mass of total ${\text{O}}$ in the formula $= 6 \times 16 = 96$
Therefore relative formula mass $= 24 + 28 + 96 = 148$

Additional information:
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.

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