help with homework question

[Kylie McCauley 1D]

Hi, can someone help me with this homework question please?

Urea is an organic compound widely used as a fertilizer. Its solubility in water allows it to be made into aqueous fertilizer solutions and applied to crops in a spray.
What is the maximum theoretical number of water molecules that one urea molecule can hydrogen bond with? Ignore shape for the purposes of this answer.

[Stuti Pradhan 2J]

There are two main requirements for forming hydrogen bonds. The first is that either an O, N, or F molecule must have a lone pair that can become the hydrogen bond. The second is that the hydrogen must be partially positive (), in order for it to be attracted to the lone pair. The partial positive charge is created due to a difference in electronegativity, and the only element I have seen in the homework questions so far that when bonded to hydrogen does not create a partial positive charge on the hydrogen is carbon (the electronegativity of hydrogen is very close to that of carbon, so there is no effective unequal sharing of electrons).

Based on these rules, urea could potentially form bonds with 8 water molecules. One water molecule could bond with each lone pair on the oxygen, for a total of 2 water molecules. One water molecule could bond with the lone pair on the nitrogen, and since there are 2 nitrogen atoms with a lone pair, this would result in the bonding of another 2 water molecules. Finally, each of the hydrogen atoms could also bond with a water molecule (the hydrogen would bond with the oxygen of the water molecule). The difference in electronegativity between the nitrogen and the hydrogen is great enough to give the hydrogen atoms in urea a partial positive charge. Therefore, each hydrogen could bond with a water molecule, and since there are 4 hydrogen atoms bonded to the nitrogen atoms, that would result in a total of 4 water molecules.

Add up all the water molecules bonding with the oxygen, nitrogen, and hydrogen atoms in urea and you should get that it is possible for the urea molecule to bond with 8 water molecules.

Hope this helps!