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JAMB Chemistry 1985 Question 46

Question 46

Sulphur and its Compounds

Addition of dilute hydrochloric acid to an aqueous solution of a crystalline salt yielded a yellow precipitate and a gas which turned dichromate paper green. The crystalline salt was probably

  1. A.Na2SO4
  2. B.Na2S
  3. C.Na2S2O3.5H2O
  4. D.Na2CO3
  5. E.NaHCO3.
Show the answer and explanation

Answer: C (Na2S2O3.5H2O)

Explanation

Acid decomposes sodium thiosulphate into a yellow precipitate of sulphur and sulphur (IV) oxide gas. SO2 turns orange dichromate paper green.