Question

A salt crystal has a mass of $\mathbf{0}\mathbf{.12}\ \mathbf{mg}.$ How many $\mathbf{Nacl}$ formula units does it contain?

Answer 1

Hint: A mole is defined as the number of atoms in $12.00\ \text{g}$ of carbon-$12$ it is found to be $6.02\times {{10}^{23}}$ . This number is also known as Avogadro’s number. A mole is a fundamental unit which helps to calculate a big number. Which is difficult to calculate in $x$ at numbers.

Complete step by step answer:
To find $\text{NaCl}$ formula units in $0.12\ \text{mg}$ of salt crystal we need to find out the number of moles of sodium chloride $\text{(NaCl)}$ in $0.12\ \text{mg}$ of salt.
As we know
\[\text{moles}=\dfrac{\text{given mass}}{\text{Molecular Mass}}\]
$=\text{Given mass}=0.12\text{ }mg=\dfrac{0.12}{1000}=0.00012\ \text{g}$
$=\text{molecular mass (mass of sodium }+\text{ mass of chlorine)}$
$=(23+35.5)\ \text{g/mol}$
Putting values]
$\text{No}\text{. of moles}=\dfrac{0.00012\text{ g}}{58.5\text{ g/mol}}=2.051\times {{10}^{-6}}\ \text{moles}$
As we know $1$ mole contains $6.02\times {{10}^{27}}$ particles.
Therefore $2.051\times {{10}^{-6}}$ moles contains $1.2\times {{10}^{18}}$ particles unit of $\text{NaCl}$
The solution of the question is $1.2\times {{10}^{18}}$ particles.
In chemistry, mole is widely used to calculate the amount of substance formed, volume of gas released at standard temperature and pressure condition called STP, number of particles or molecules formed during a chemical reaction.

Note: The number of units in one mole of any substance is called Avogadro’s number. It is equal to $6.022\times {{10}^{23}}.$ The units may be electrons, ions, atoms or molecules.
Mole can also be find out by many ways
\[\text{Number of moles--}\dfrac{\text{mass of given atom}}{\text{gram atomic mass}}\]
\[\text{Number of moles--}\dfrac{\text{mass of given molecule}}{\text{gram molecular mass}}\]
\[\text{Number of moles--}\dfrac{\text{Given volume of gas at STP}}{\text{molar volume of }\!\!~\!\!\text{ 22}\text{.4 litre}\text{.}}\]