Question

A salt crystal has a mass of 0.11 mg. How many NaCl formula units does it contain? Express your answer using two significant figures.

Answer 1

Answer: The number of formula units in the given amount of NaCl are $1.1\times 10^{18}$

Explanation:

To calculate the number of moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$

Given mass of NaCl = 0.11 mg = $1.1\times 10^{-4}g$      (Conversion factor:  1 g = 1000 mg)

Molar mass of NaCl = 58.5 g/mol

Putting values in above equation, we get:

$\text{Moles of NaCl}=\frac{1.1\times 10^{-4}g}{58.5g/mol}=1.88\times 10^{-6}mol$

According to mole concept:

1 mole of a compound contains $6.022\times 10^{23}$ number of formula units

So, $1.88\times 10^{-6}mol$ moles of NaCl will contain = $(1.88\times 10^{-6}\times 6.022\times 10^{23})=1.1\times 10^{18}$ number of formula units

Hence, the number of formula units in the given amount of NaCl are $1.1\times 10^{18}$