12.4 Unknown Metal lons- Post-Lab Questions Name Date: Instructor Section Group
ID: 589501 • Letter: 1
Question
12.4 Unknown Metal lons- Post-Lab Questions Name Date: Instructor Section Group Show all work for full credit.(Note: td.-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 (td. OH-) and dilute aqueous NH3 produce similar results, why do the results differ between excess NaOH (xs.OH") and concentrated aqueous NH3? 3. Identity the following ions based upon their observed reactions. The original color of the un-reacted solutions was not recorded Anion so,2 sa Unknown B no reaction black ppt. 2 produced orange brown ppt. more ppt. formed orange brown ppt more ppt. formed Unknown A no reaction black ppt no reaction Unknown C no reaction black ppt ltd. OH xs. OH Itd. NH3 xS. NH3 dark blue ppt more ppt. formed dark blue ppt clear soln. produced It. blue ppt. more ppt. formed It. blue ppt royal blue soln.Explanation / Answer
1 &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.
3)
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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