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

Nov 4, 2016

We need (i) the molar quantity of $\text{sodium chloride}$........

#### Explanation:

$\text{Number of moles}$ $=$ $\text{Mass"/"Molar mass}$

$=$ $\frac{0.15 \times {10}^{-} 3 \cdot g}{58.44 \cdot g \cdot m o {l}^{-} 1}$ $=$ $\text{?? moles}$

And (ii) we need the number of sodium chloride units. Now it is a fact that $\text{Avogadro's number, } {N}_{A}$ $\equiv$ $6.022 \times {10}^{23} \cdot m o {l}^{-} 1$

And thus we mulitply the number of moles by $\text{Avogadro's number}$ to give the number of sodium chloride formula units:

$\frac{0.15 \times {10}^{-} 3 \cdot g}{58.44 \cdot g \cdot m o {l}^{-} 1} \times 6.022 \times {10}^{23} \cdot m o {l}^{-} 1$, and we get the answer as a number, as required.