Question

Calculate the formula unit mass (u) of NaCl [Na = 23 u, Cl = 35.5 u]?
${\text{A}}{\text{.}}$ 54.5 u
${\text{B}}{\text{.}}$ 55.5 u
${\text{C}}{\text{.}}$ 56.5 u
${\text{D}}{\text{.}}$ 58.5 u

Answer 1

Hint- Here, we will proceed by defining the formula unit mass of any substance. Then, we will mention the procedure for calculating the formula unit mass of any general ionic compound ${{\text{A}}_x}{{\text{B}}_y}{{\text{C}}_z}$.

Complete answer:
Given, Atomic mass unit of Na = 23 u
Atomic mass unit of Cl = 35.5 u
The formula unit mass of a substance is defined as the sum of the atomic masses of constituent atoms in an ionic compound. This is generally used for ionic compounds that do not contain individual molecules but rather ions as their constituent units.
The simple procedure of calculating the formula unit mass of any ionic compound having a general chemical formula ${{\text{A}}_x}{{\text{B}}_y}{{\text{C}}_z}$ is given under.
Firstly, identify all the constituent atoms or units of which the given ionic compound comprises. Then, write down the atomic masses of these constituent atoms or units according to that written in the defined periodic table.
Clearly, the ionic compound consists of x atoms of A, y atoms of B and z atoms of C.
So, the formula unit mass will be equal to the sum of all the products obtained by multiplying the number of atoms of a constituent with their corresponding atomic mass for that particular constituent.
i.e., Formula unit mass of ${{\text{A}}_x}{{\text{B}}_y}{{\text{C}}_z}$ = x(Atomic mass of A) + y(Atomic mass of B) + z(Atomic mass of C)
Using the above formula, we can write
Formula unit mass of NaCl = Atomic mass unit of Na + Atomic mass unit of Cl
$ \Rightarrow $ Formula unit mass or Atomic mass unit of NaCl = 23 + 35.5 = 58.5 u
Therefore, the formula unit mass (u) of NaCl is 58.5 u

Hence, option D is correct.

Note- In order to differentiate substances like NaCl, which does not contain individual molecules, some chemists refer to their formula unit instead of the molecular mass or molecular weight, they use the term formula mass or formula unit mass. The formula mass unit is calculated in the same manner as the molecular mass is calculated.