NCERT Question 10 - Chapter 3 Class 9 - Atoms And Molecules

Calculate the number of molecules of sulphur (S 8 ) present in 16 g of solid sulphur.

Answer

We know that,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

                                                                n  = m/M ----- (1)

Also,

Number of moles of a substance = Number of particles of the substance / Avogadro's Constant

 n  = N / N _O

n  = N / 6.022 x 10 23 ------- (2)

From the above 2 formulae, we can say that,

 n = m / M = N / 6.022 x 10 23   ---- (3)

Given mass of sulphur molecule(S ₈ ) = m = 16g

Molar mass of Sulphur Molecule (S ₈ ) = M = 256g

We need to find Number of molecules, ie, N

Putting values in (3), we get

16 / 256 = N / 6.022 x 10 23  

N = 6.022 x 10 23   x (16 / 256)

   = 6.022 x 10 23   / 16

   = 3.011 x 10 23 /8

   = 0.37 x 10 23 molecules of Sulphur(S ₈ )

   = 3.7 x 10 22 molecules of Sulphur (S ₈ )