Question: Phosphoric acid is the conjugate base of which phosphate ion?
Paraphrase: Phosphoric corrosive is the conjugate base of which phosphate ion?
SPARQL (Wikidata):  select distinct ?obj where { wd:Q184782 wdt:P4149 ?obj . ?obj wdt:P31 wd:Q55168228 } 
Answer: dihydrogenphosphate ion