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.