Question

A salt crystal has a mass of 0.15 mg. How many NaCl formula units does it contain?

Answer 1

The sodium chloride is an ionic compound where the sodium is cation and chloride is anion and both the ions are present in the same proportion. The formula unit of sodium chloride is NaCl and it is obtained by multiplying the number of moles with the Avagadro’s number.

Complete step by step answer: The number of formula units is calculated by the formula as shown below.
$F = \dfrac{m}{M} \times {N_A}$
Where,
F is the formula unit
m is the mass of the compound
M is the molecular weight of the compound.
${N_A}$ is the avogadro's number
The molecular weight is defined as the sum of the atomic weight of the atoms present in the compound where the number of atoms (if present) is multiplied with the atomic number.
The chemical formula of sodium chloride is NaCl which is formed from sodium ion and chloride ion.
The atomic weight of sodium is 22.989
The atomic weight of chloride is 35.453
The molecular weight of NaCl is calculated as shown below.
$\Rightarrow Molecular\;weight = 22.989 + 35.453$
$\Rightarrow Molecular\;weight = 58.4427g/mol$
The Avagadro’s number is the unit of one mole of substance which is equal to $6.022 \times {10^{23}}$ or one mole of substance contains $6.022 \times {10^{23}}$
To calculate the formula unit, substitute the values in the given formula.
$\Rightarrow F = \dfrac{{\dfrac{{0.15}}{{1000}}}}{{58.4427gmo{l^{ - 1}}}} \times 6.022 \times {10^{23}}mo{l^{ - 1}}$
$\Rightarrow F = 1.54 \times {10^{18}}$
Therefore, a salt crystal has a mass of 0.15 mg contains $1.54 \times {10^{18}}$ formula unit of NaCl

Note:
The molecular weight of the compound is calculated in terms of grams per mole and the given mass is in milligrams, so make sure to convert the value in milligrams into grams. $1mg = \dfrac{1}{{1000g}}$.