Hydrogen bonding between water and other molecules.

[Puchinita5]

Homework Statement

I have attached a solution to one of my homework questions that states "Draw hydrogen bonds that might be formed between a water molecule and the following compounds: a) H3PO4; b) PO43-; c) aniline; d) pyridine; e) ethylamine; f) ethyl ammonium ion; g) acetic acid."

My question is specifically for H3P04 and Acetic acid. Why is it that in H3PO4, the H on the OH makes the hydrogen bond with the O of a water molecule, but in acetic acid, the lone electron pair on the OH makes the hydrogen bond with the H of a water molecule.

Does it make a difference if I instead drew a hydrogen bond from the lone electron pair of the OH on H3PO4 and connected this to the H of water? Is it the same thing or is there a reason these are like this?

Homework Equations

The Attempt at a Solution

 

[AGNuke]

A Polarized Hydrogen atom makes Hydrogen bond with Polarized F, O, N and to some extent, Cl.

Water molecule will form Hydrogen Bonds from both Oxygen and Hydrogen atoms as the species you mentioned, Phosphoric Acid and Acetic Acid, have both polarized O and H atom.

But I feel that, in Acetic Acid, Water must form H-Bond from Doubly Bonded Oxygen atom and Acidic Hydrogen, as the O atom in C=O is more electronegative than O atom in C-O-H, hence having more electron density and tendency to attract partially positive H atom of water molecule.

If there are polarized H and F/O/N atom, you can draw H-Bond with both of them, which you should.