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

Dec 22, 2015

#### Answer:

$1.2 \times {10}^{18}$

#### Explanation:

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

$\therefore n = \frac{m}{M r} = \left(\frac{0.12}{1000} \text{g")/((23+35.5)"g/mol}\right) = 2.051 \cdot {10}^{- 6}$ $\text{moles}$

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

Therefore by ratio and proportion, $2.051 \cdot {10}^{- 6}$ $\text{mol}$ will contain $1.2 \times {10}^{18}$ particles of $\text{NaCl}$.