12.4 Unknown Metal lons -Post-Lab Questions Name: Date: Instructor: Section/Grou

Question

12.4 Unknown Metal lons -Post-Lab Questions Name: Date: Instructor: Section/Group: Show all work for full credit. (Note: ltd.·limited, xs: excess) 1. Why does reaction of the metal ions with dilute NaOH (limited OH") produce the same results as the reaction with dilute aqueous NH3? 2. Since dilute NaOH (d. O--) and dilute aqueous NHig produce similar results, why do the resulits difer between excess NaOH (bxs. OH-) and concentrated aqueous NH3? 3. Identily the following ions based upon their observed reactions. The original color of the un-reacted solutions was not recorded. Anion so,2 s2- Unknown A no reaction black ppt. no reaction dark blue ppt more ppt. formed dark blue ppt Unknown B no reaction black ppt. 2 produced orange brown ppt more ppt. formed orange brown ppt more ppt. formed Unknown C no reaction black ppt. td. OH xs. OH td. NH3 xs. NH3 It. blue ppt more ppt. formed It. blue ppt royal blue soln. clear soln

Explanation / Answer

1. and 2. As NaOH is a strong base it completely dissociates in aqueous solution giving an equal concentration of [OH] ion in the solution whereas NH3 in aqueous solution becomes NH4 OH which is a weak Arrhenius base and it does not completely dissociate in the solution giving a limited concentration of [OH] ion in solution. Therefore, the reaction on metals produces similar results when limited NaOH is used with dilute NH4OH and different results when excess NaOH is used with concentrated NH4OH.

A will be Ag+

B will be Fe3+

C will be Cu 2+

Since all these ions from black precipitate with Sulphide ions. Fe3+ forms orange-brown ppt with hydroxide ions and Cu 2+ forms light blue coloured oil gel. Further, Ag+ forms complex with excess ammonia and hence is soluble giving a clear solution. Ag+ ion does not produce iodine as it forms a ppt with the same.

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