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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 16.73

When a few drops of concentrated Na2SO4(aq) are added to a dilute solution of AgNO3(aq), a white precipitate forms. When a small quantity of concentrated NH3(aq) is added to the mixture, the precipitate redissolves, resulting in a colorless solution. When this solution is made acidic with HNO3(aq), a white precipitate again appears. Write equations to represent these three observations.




Answer:

Observation 1:

Na2SO4(aq) + 2 AgNO3(aq) Ag2SO4(s) + 2 Na+(aq) + 2 NO3(aq)

The white precipitate is silver sulfate.

Observation 2:

Ag2SO4(s) + 4 NH3(aq)2 [Ag(NH3)2]+(aq) + SO42–(aq)

The complex ion formation causes the silver sulfate to dissolve.

Observation 3:

2 [Ag(NH3)2]+(aq) + SO42–(aq) + 4 HNO3(aq) Ag2SO4(s) + 4 NH4+(aq) + 4 NO3(aq)

The acid/base reaction between nitric acid and ammonia causes dissociation of the complex ion, leaving the silver sulfate to reprecipitate.