I guess the reason is hydrogen bonding, but shouldn't both the trends be similar in that case?

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    $\begingroup$ Boiling point is simple to predict. Melting point is tricky. $\endgroup$ Aug 16, 2019 at 15:37
  • $\begingroup$ @IvanNeretin I think even the boiling point of ammonia should be more than HF as it can form 4 hydrogen bonds while the later only 2 $\endgroup$
    – Stan
    Aug 16, 2019 at 16:47
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    $\begingroup$ You think wrong. Ammonia is no different from HF in this regard. $\endgroup$ Aug 16, 2019 at 16:53
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    $\begingroup$ @Stan That doesn't quite make sense because ammonia molecules can only accept 1 hydrogen bond. You have them all form 3 hydrogen bonds via their N-H's because there aren't enough acceptors. $\endgroup$
    – Zhe
    Aug 16, 2019 at 17:19
  • $\begingroup$ You are comparing pears and apples. Two very different molecules, two very different substances. There isn't one reason why their melting and boiling points are what they are. At the very qualitative level you are looking at it, their Mp is the same. And the Bp of HF is significantly higher because its hydrogen bonds are much stronger. $\endgroup$
    – Karl
    Aug 16, 2019 at 19:37