June 2017 NY Regents Chemistry Exam
34 What is the chemical formula for sodium sulfate?
(1) Na2SO4
(2) Na2SO3 (3) NaSO4
(4) NaSO3
Solution: Sodium sulfate is an ionic compound that consists of a metal (sodium) and a polyatomic ion, sulfate. Table E on the Reference Table lists common polyatomic ions with their names, formulas and oxidation numbers. Sulfate is SO4 ^2-. The oxidation number for sodium can be found on the Periodic Table and is +1.
Putting the name together we write both parts with their oxidation numbers and then we criss cross the numbers and make them into subscripts.
Na^+1 SO4^2-
Na2SO4
Answer: 1
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