Question

The formula unit mass of $C{a_3}{\left( {P{O_4}} \right)_2}$ is:
(i) $360$
(ii) $240$
(iii) $118$
(iv) $310$

Answer 1

Hint:The mass of the formula unit of a substance is the sum of the atomic masses of all atoms in an ionic compound's formula unit. In the given question find out the unique elements present in the compound multiply them with their atomic masses. Finally add all to get the formula unit mass of the compound.

Complete step-by-step answer:The sum of the atomic masses of constituent atoms in a given ionic compound is known as the formula mass of the substance. This is true for ionic compounds which contain ions as their constituent units and not discrete molecules.
Formula unit mass is generally measured in $u$. 
In the question, $C{a_3}{\left( {P{O_4}} \right)_2}$is given. First, we need to find out the different types of elements present in the compound along with their respective atomic masses. They are:
$ \bullet $Calcium $\left( {Ca} \right)$ having an atomic mass of $40\,u$
$ \bullet $Phosphorus $\left( P \right)$having an atomic mass of $31\,u$
$ \bullet $Oxygen $\left( O \right)$having an atomic mass of $16\,u$.
Now the formula unit mass of $C{a_3}{\left( {P{O_4}} \right)_2}$ is given by
$ = \,($Number of $Ca$atoms present in the compound$ \times $Atomic mass of $Ca)\, + \,($Number of $P$atoms present in the compound$ \times $Atomic mass of $P)\, + \,($$ = \,($Number of $O$atoms present in the compound$ \times $Atomic mass of $O)$
Now, Number of $Ca$atoms present in the compound $ = \,3$
Number of $P$atoms present in the compound $ = \,2$
Number of $O$atoms present in the compound$ = \,8$.
Therefore, formula unit mass of $C{a_3}{\left( {P{O_4}} \right)_2}\,\, = \,\,\left\{ {\left( {3 \times 40} \right) + \left( {2 \times 31} \right) + \left( {8 \times 16} \right)} \right\}\,u$
$ = \,\left( {120 + 62 + 128} \right)\,u\,\,\, = \,\,310\,u$

Hence the correct answer is (iv) $310$.

Note:For these kinds of questions it is better to know the periodic table and the position of the different elements in the periodic table properly. If you know the different position of the atomic masses of the elements will also be known to you and hence you can determine the formula unit mass of a given compound.