Equal volumes of ammonia gas and chlorine gas are kept in two different containers under the same conditions of temperature and pressure.
Find the number of molecules contained in chlorine gas when the mass of ammonia is 34 g.
[Atomic weight: Cl=35.5, H=1, N=14]
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According to Avogadro's Law, equal volumes of all gases under the same conditions of temperature and pressure contain the same number of molecules.
Hence, the number of molecules of chlorine gas = number of molecules of ammonia gas.
Molar mass of NH3 = 14 + 3(1) = 17 g
Number of moles of NH3 = $\frac{\text{Mass}}{\text{Molar mass}}$ = $\frac{34}{17}$ = 2 moles
Number of molecules in 1 mole = 6.022 × 1023
∴ Number of molecules in 2 moles of NH3 = 2 × 6.022 × 1023 = 1.2044 × 1024 molecules
Hence, the number of molecules in chlorine gas = 1.2044 × 1024 molecules.