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

The molecular mass of an element is the sum of the atomic masses of all its constituent elements. This quantity is also represented in terms of atomic mass units.

To calculate the number of molecules in a substance

The total number of atoms/molecules in a sample can be calculated by multiplying the number of moles with the Avogadro constant. This formula can be written as:

Number of Atoms or Molecules = (Number of Moles)*(6.022*1023)

Example

To calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
1 mole of solid sulphur (S8) = 8 x 32 g = 256

256 g of solid sulphur contains = 6.022 x 1023 molecules
Then, 16 g of solid sulphur contains = 6.022 x 1023 / 256 = 16 molecules
= 3.76375 x 1022 molecules

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