Question

Calculate the formula unit mass of $KCl$ and $Ca{{\left( N{{O}_{3}} \right)}_{2}}$?

Answer 1

Hint: For solving this problem, we need to understand the meaning of formula unit mass. It is the total of the relative atomic masses of the atoms in the number appeared in the formula. It is represented by the symbol ${{M}_{r}}$.

Complete step-by-step answer:
To calculate the formula unit mass, find out the number of atoms of every element present in the chemical formula.
Add together the relative atomic mass values for all the atoms of every element present.
Let us calculate the formula unit mass of $KCl$. We know that it contains one atom of potassium and one atom of chlorine in the chemical formula.
Now, we know that:
Atomic mass of potassium is $39.0$ amu
And Atomic mass of chlorine is $35.5$amu
Therefore, formula unit mass of potassium chloride can be calculated as
$\Rightarrow 39+35.5=74.5$amu
Hence the formula unit mass of potassium chloride is $74.5$amu.

Now let us calculate the formula unit for $Ca{{\left( N{{O}_{3}} \right)}_{2}}$. We can see that this compound consists of one calcium atom, two nitrogen atoms and six oxygen atoms.
Now, we know that
Atomic mass of calcium is $40.0$amu
Atomic mass of nitrogen is $17.0$amu
Atomic mass of oxygen is $15.9$amu
Therefore, formula unit mass of calcium nitrate can be calculated as
$\Rightarrow 40+2\left ( 17.0 \right)+6\left( 15.9 \right)=169.4$amu

Hence, the formula unit mass of calcium nitrate is $169.4$ amu.

Note: Atoms have next to no mass so their relative atomic masses are used. The relative atomic mass of an element, symbol ${{A}_{r}}$is the relative mass of its atoms contrasted with the mass of carbon-12 atoms. It is always advisable to learn the molar mass of a few basic elements.