The following gaseous reactions are started from the pure reactant. At constant temperature and pressure, for which does the vapour density of the mixture become smaller than that of the pure reactant as the reaction proceeds? (Vapour density here is $M_{avg} /2$ .)

Answer: A, C, D

Average molar mass falls whenever the moles of gas increase on reaction (

Δ n_{g} > 0

+ 1

for

PCl X_{5}

+ 1

for

N X_{2} O X_{4}

+ 2

for

2 NH X_{3}

. For

2 HI H X_{2} + I X_{2}

Δ n_{g} = 0

, so the mole count and hence average molar mass are unchanged.