Question: What is the polyatomic anion for the conjugate acid of water?
Paraphrase: For the conjugate acid of water, what's the polyatomic anion?
SPARQL (Wikidata):  select distinct ?sbj where { ?sbj wdt:P4147 wd:Q283 . ?sbj wdt:P31 wd:Q55511438 } 
Answer: hydroxide ion