How many hydrogen bonds are possible for $\ce{H2O}$, given that oxygen has two lone pairs? Is it 4 or 2?

Related: why is it that HF forms only 1 hydrogen bond, given that HF has three lone pairs?

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    $\begingroup$ Try drawing a bunch of water molecules. How many ways do they have to connect to others? $\endgroup$ – M.A.R. Feb 27 '17 at 6:43
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    $\begingroup$ If you search online for water hydrogen bonding you will find many images that will answer your question. Same for HF. $\endgroup$ – porphyrin Feb 27 '17 at 8:37

Water is capable of participating in 4 hydrogen bonds at once, granted it only does this when it forms a perfect crystal structure. That means that every hydrogen will coordinate to an oxygen lone pair. In the liquid phase, the average number of hydrogen bonds that a water molecule participates in varies greatly with temperature, the average being 3.69 at 0ºC, 3.59 at 25ºC, and decreasing to 3.24 at 100ºC as a result of increased molecular motion and decreased density.$^{[1]}$

Hydrogen fluoride is also capable of participating in 4 bonds at once, but in the solid state forms only 2 hydrogen bonds on average. Though it has three fluorine lone pairs, each of which is capable of accepting a hydrogen bond, there are not enough hydrogen for every fluorine to do so. Similarly, ammonia can also form 4 hydrogen bonds but only forms 2 on average because each molecule has only one lone pair.

$^{[1]}$ Hydrogen Bond, Hydrogen bonds in water

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