Question

Concentrated orthophosphoric acid is a syrupy, viscous liquid and this is due to __ bonding.

Answer 1

Let us take a look at the structure of $H_{3}P{O}_4$:
As we can see from the above structure, the Oxygen atom shown to have double bond character acts as a site to accept Hydrogen bonds while the other Hydrogen atoms attached via a single bond to Oxygen atoms can participate in Hydrogen bond. So in all, for every molecule there are 3 donor sites and one acceptor site.