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An unknown solid mixture contains one or two of the following: CaCO3, BaCI2, AgNO3, Na2SO4, ZnSO4 and NaOH. The mixture is completely soluble in water and the solution gives pink colour with phenolphthalein. When dilute hydrochloric acid is gradually added to the above solution, a precipitate is produced which dissolves with further addition of the acid. What is/are present in the solid? Give equations to explain the appearance of the precipitate and its dissolution.

7 years ago
(i) Since the mixture is soluble in water to give strong alkaline solution, it must contain NaOH as one of the constituents.
(ii) Since the aqueous solution gives precipiteate with dil. HCI, which dissolves in excess of dil. HCI, it must contain zinc salt.
Thus the mixture consists of ZnSO4 and NaOH which explains all the given reactions.
Reactions :
(i) ZnSO4 + 4NaOH →$\underbrace{Na_{2}ZnO_{2} + Na_{2}SO_{4}}_{soluble}$+ 2H2O
(ii) Zn2SO4 2HCI → 2NaCI + Zn(OH)2
Zn(OH)2 + 2HCI → ZnCI2 + 2H2O