just take H2O AND CH3CH2OH as example.why say that h2o can form two hydrogen bond per molecule but CH3CH2OH only one.i am confused as both of them also has two lone pairs per molecule,is it true that one hydrogen atom will result in only one hydrogen bond and two hydrogen bond result in two hydrogen bond?

http://www.chemguide.co.uk/atoms/bonding/hbond.html

The ability of a molecule to form hydrogen bonds is determined by two factors: the presence of hydrogen atoms bonded to highly electronegative atoms (such as oxygen, nitrogen, or fluorine) and the presence of lone pairs of electrons on these electronegative atoms.

In the case of water (H2O), it has two hydrogen atoms bonded to an oxygen atom. Oxygen is highly electronegative, meaning it strongly attracts electrons towards itself. Oxygen also has two lone pairs of electrons. Due to these characteristics, each hydrogen atom in water can form a hydrogen bond with a neighboring electronegative atom (either another oxygen atom or a nitrogen atom, for example). This results in a total of two hydrogen bonds per water molecule.

On the other hand, in ethanol (CH3CH2OH), there is one hydrogen atom bonded to an oxygen atom (as part of the -OH group), and this oxygen has two lone pairs of electrons. However, the other oxygen atom in the structure, which is part of the ethyl group (CH3CH2-), does not have any hydrogen atoms bonded to it. Therefore, only one hydrogen bond can be formed with ethanol molecule, involving the hydrogen from the -OH group and an electronegative atom from a neighboring molecule.

To summarize, the presence of one hydrogen atom in ethanol capable of forming hydrogen bonds limits the molecule to only one hydrogen bond, while water has two hydrogen atoms available, allowing it to form two hydrogen bonds per molecule.