There's no disposal for waste atoms so this will have to be a perfectly balanced reaction. Phosphorus (15) can be fused of oxygen and hydrogen (8 + 7H) or out of hydrogen alone (15H). Which way makes sense?
The acid needs four oxygens and three hydrogens. That means it needs at least four water molecules, as each water molecule contains one oxygen. That leaves a remainder of five hydrogen atoms.
All the atoms in one water molecule plus five hydrogen atoms all fused together makes phosphorus. This means that five water molecules is precisely enough to build one acid molecule.
Stripping one hydrogen from each water molecule would be a good start.
Note that oxygen has a maximum of two bonds. If the phosphorus tries to double bond itself to a hydroxyl radical (OH), it will only form a single bond due to max bonds on the oxygen atom.
This is a decent pipeline.
The legend which shows OH and oxygen outputs three OH and one oxygen in a sequence.
The reactor that outputs neon and the OH and oxygen is slightly tricky to design.
This design uses the blue waldo as a worker that moves atoms from the alpha input to the omega output. Note that it doesn't input any atoms itself because that would require twice as much syncronization due to the blocking nature of inputs. The red waldo first fuses an OH together to get fluorine. Then it inputs another and fuses again to get neon. This is sent to the output to the next reactor, where it is spammed with five hydrogen to create phosphorus. It also sends the remaining oxygen atom away and it creates three additional OH molecules and sends them away using the blue waldo. This creates the correct sequence to be bonded to the phosphorus.
To create the acid is simpler than you may think:
The phosphorus is brought down in to a drive-thru loop for bonding the components around it. Note that is double bonds whether it needs it or not. It rotates each time and eventually the sensor picks up that there is an oxygen already present on the bonding side, in which case all four sides of the phosphorus atom have been bonded and the acid is complete.