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why HF has lower boiling point than H2O?

In a molecule of water there are two hydrogen bond donors, while a HF molecule only has one donor. There is only one hydrogen in HF, but there are 2 hydrogen atoms in water. The oxygen molecule in water has 2 lone pairs which is capable of accepting two hydrogen bonds. Therefore a single water molecule can have up to 4 hydrogen bonds i.e. 1 for each hydrogen and 2 for the oxygen. HF on the other hand can only undergo 2 hydrogen bonds per molecule. Thus it is clear that water has more hydrogen bonds per molecule and per molecular mass as compared to HF. This allows water to form a complex network of hydrogen bonding that creates a greater surface tension and consequently a higher bp than HF.

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By the way we all know, why H2O,NH3,HF has significantly higher boiling point than the other period members, yes, because of HYDROGEN BONDING. It is obvious that, more the electronegative element the H attached to, the more will be its magnitude of H Bond. So it the hydrogen-bond strength follows order HF> H2O>NH3. Still, H2O has higher boiling point than HF, the reason is as follows.
HF is a linear molecule, a molecule of HF can only form 2 hydrogen bonds (as ---H-F---H-F---H-F---). But the shape of water molecule is 'V Shaped' due to the presence of 2 lone pairs(of course F in HF also has lone pairs, but it cant utilize it for H-bond due to its linear shape), and a molecule of H2O can form 4 hydrogen bonds(where HF there is only 2). Thus eventhough bond strength is high in HF, the combined bond strengths of 4 Hydrogen Bonds in H2O will overcome the combined bond strength of 2 Hydrogen bonds in HF. Thus H2O has more boiling point than HF...
Hope you're clear!

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