How do you calculate the number of molecules in a substance?

Step 1: Calculate the number of moles in a substance:-

The mole is used as a measurement or a basic unit that helps in calculating the amount of substance present in the given sample.

To calculate the number of molecules, one must first calculate the number of moles of a given element.

Here, the number of moles can be calculated by the ratio of the given mass to the molar mass.

Step 2: Calculate the number of molecules in a substance using no of moles:-

For calculating the number of molecules, the relation between the number of moles and Avogadro’s number can be used, which is given by;

1 mole of a number =Avogadro’s constant ${\mathrm{N}}_{\mathrm{A}}$.

After calculating the number of moles, the number of molecules will be equal to the product of the number of moles and Avogadro’s number, given by;

Number of ions= (number of moles)X ${\mathrm{N}}_{\mathrm{A}}$$(6.022\times {10}^{23})$ ( $\therefore {\mathrm{N}}_{\mathrm{A}}$= Avogadro’s number)

Step 3: Example:-

Calculating the number of molecules of Sulphur$\left({\mathrm{S}}_8\right)$ present in $16\mathrm{g}$ of Solid Sulphur:
1 mole of solid sulfur $\left({\mathrm{S}}_8\right)$=$8\mathrm{x}32\mathrm{g}=256\mathrm{g}$

256 g of solid sulfur contains = ${\mathrm{N}}_{\mathrm{A}}$$(6.022\times {10}^{23})$molecules.
16 g of solid sulphur contains

= $\frac{(6.022\times {10}^{23})}{256}\times 16\mathrm{molecules}$
= $3.76375\mathrm{x}{10}^{22}$ molecules

Hence, the number of molecules in a substance can be calculated as above, using the mole concept as; $\mathrm{Number}\mathrm{molecules}=(\mathrm{number}\mathrm{of}\mathrm{moles})\times {\mathrm{N}}_{\mathrm{A}}$