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Arrange the hydrides of the p-Block Group 15 elements in order of increasing boiling point:

$\text{NH}_3 \gt \text{PH}_3 \gt \text{AsH}_3 \gt \text{SbH}_3 \gt \text{BiH}_3 $ $\text{NH}_3 \gt \text{PH}_3 \lt \text{AsH}_3 \lt \text{SbH}_3 \gt \text{BiH}_3 $ $\text{NH}_3 \gt \text{PH}_3 \lt \text{AsH}_3 \lt \text{SbH}_3 \lt \text{BiH}_3 $ $\text{NH}_3 \lt \text{PH}_3 \lt \text{AsH}_3 \lt \text{SbH}_3 \lt \text{BiH}_3 $

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Answer: $\text{NH}_3 \gt \text{PH}_3 \lt \text{AsH}_3 \lt \text{SbH}_3 \lt \text{BiH}_3 $
From $ \text{PH}_3$ to $\text{BiH}_3 $, the boiling points increase with increase in molecular weights, due to the increase in Van der Waal's forces with molecular mass.
However, boiling point of $\text{NH}_3 \gt \text{PH}_3 $ even though $\text{NH}_3 $ has lower molecular mass than $\text{PH}_3$, due to the intermolecular hydrogen bonding in Ammonia that is not present in $\text{PH}_3$ and other hydrides in Group-15.
answered Jul 14, 2014 by balaji.thirumalai

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