A gaseous mixture contains NH3 and H2S gases is 20% H2S by moles. The Vapour density of the mixture is
20.2
10.2
15.3
30.6

Mixture contain 20 % of H2S by mole. Means in 1 mole mixture there is 0.2 mole H2S and 0.8 mole NH3.
At first calculate the Molar mass (MM) of mixture.
MM of mixture = (% of H2S x MM of H2S) + (% of NH3 x MM of NH3)
                         = 0.2 x 34 g/mol + 0.8 x 17 g/mol
​                         = 20.4 g/mol
​Vapour density = Molar mass of mixture of gases/2
                        = 20.4 g/mol / 2
                       = 10.2
 

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