In Step 34 of the procedure, a student was assigned a colorless unknown. The stu

Question

In Step 34 of the procedure, a student was assigned a colorless unknown. The student added five drops of 1.0 M barium chloride solution to 1.0 mL of the unknown solution. A white precipitate formed. In a second experiment, the student added two drops of 0.5 M NH_3 solution to 1.0mL of the unknown solution. A white precipitate formed. The student added five drops of 4.0 M NH_3 to the resulting solution in the second experiment. The white precipitate did not dissolve. Identify the colorless unknown solution. Justify your answer with an explanation. Write the net ionic equations for all reactions.

Explanation / Answer

The colourless solution is aluminium sulphate Al2(SO4)3 .

Barium chloride gives a white precipitate on reaction with sulphate ions ((SO4)2-) in water producing barium sulphate (which is not soluble in water resulting in precipitation) .

Al3+ , Zn2+  ions gives white precipitate in small concentrtion of NH3 solutions and but on further addition of more concentrated solution of NH3 the solution with Zn2+ the precipitate dissolves and for solution with Al3+ no change takes place . So as given the metal ion is Al3+ .

Net ionic equations(it doesnt show spectator ions) : Ba2+ (aq)+ (SO4)2-(aq) --> BaSO4(s)

Al3+ (aq) + 3OH- (aq) -----> Al(OH)3 (s)

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