Question

A salt crystal has a mass of 0.12 mg. How many NaCl formula units does it contain?

Answer 1

First, find the number of moles of sodium chloride in the 0.12 mg by using the formula moles = mass/molar mass.

$\therefore n=\frac{m}{Mr}=\frac{\frac{0.12}{1000}}{(23+35.5)g/mol}={\mathrm{2.051.10}}^{-6}moles$

But, 1 mol contains an Avogadro number of particles, ie $6.022\times {10}^{23}$ particles.

Therefore by ratio and proportion, ${\mathrm{2.051.10}}^{-6}moles$ will contain $1.2\times {10}^{18}$ particles of NaCl.