January 2020 NY Regents Chemistry Exam
36 What is the chemical formula for sodium oxalate?
(1) NaO (2) Na2O
(3) NaC2O4
(4) Na2C2O4
Solution: Sodium oxalate is an ionic compound because it is composed of a metal (sodium) and a polyatomic ion (oxalate). Table E lists polyatomic ions. Oxalate is C2O4^2–. The oxidation number (charge) of sodium ion can be found on the Periodic Table and is +1. The charge numbers get exchanged to make the correct ionic formula.
Na^+1 C2O4^2–
Na2(C2O4)1 = Na2C2O4
Answer: 4
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