What would be the written names for these compounds: `K_3PO_4` and `HCl` ?

The names for these compounds are:

Tripotassium phosphate, `K_3PO_4`  

and, hydrochloric acid, HCl

Lets look at each of them in more detail. The first compound contains 3 potassium ions (`K^+` , potassium is denoted by symbol K) and 1 phosphate group (P is for phosphorus and O represents oxygen). The Phosphate group is `PO_4^(3-)` . So, the compound that contains 3 potassium ions and 1 phosphate ion is designated as tripotassium phosphate (tri for 3).

The second compound contains 1 ion each of hydrogen (`H^+` ) and chloride (`Cl^-` ). The hydrogen ion, before the chloride ion, is the clue that this is an acid. Sulfuric acid (`H_2SO_4` ) is another similar example of acid (and its naming). Since, it is an acid and has chlorine in it, it is named as hydrochloric acid.

Hope this helps.