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Question

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


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Solution

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 NA.

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 NA(6.022×1023) ( NA= Avogadro’s number)

Step 3: Example:-

Calculating the number of molecules of Sulphur(S8) present in 16g of Solid Sulphur:
1 mole of solid sulfur (S8)=8x32g=256g

256 g of solid sulfur contains = NA(6.022×1023)molecules.
16 g of solid sulphur contains

= (6.022×1023)256×16molecules
= 3.76375x1022 molecules

Hence, the number of molecules in a substance can be calculated as above, using the mole concept as; Numbermolecules=(numberofmoles)×NA


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