Post-Laboratory Questions (use the spaces provided for the answers and additiona
ID: 1019962 • Letter: P
Question
Post-Laboratory Questions (use the spaces provided for the answers and additional paper if necessary.) 1. Suppose that a solution is tested for NO2" ion by adding 3M H,SO, and heating, repeating this process until no further reaction occurs. This solution is then analyzed for NO, ion by adding FeSO4 solution and more H2SO, heating the test tube and its contents. The NOs ion reaction is positive. In this case, is it accurate to state that NOs ion was present in the original solution? Briefly explain. Write appropriate equations to support your answer. 2. A solution contains a mixture of Cl and Br ions. Can both be positively identified? Briefly explain. Write appropriate equations to support your answer 3. The solubility product constants of Agcl, AgBr, and Agl are, respectively, 1.7 x 10-10,4.1 x 10-13, and 1.5 × 10-16 The Ksp expression for each silver halide, AgX, is: When the ion product, IAg ]IX 1, exceeds the Ksp, AgX(s) will precipitate until the ion product once again equals Kyp If the Ag ion concentration in the mixture is again equals Kap. If the Ag ion concentration in the mixture is 0.067M (diluted from 0.1M), calculate the detection limit of this reaction for each halide, the value below which there will be no observed reaction.
Explanation / Answer
1. For NO2-, addition of H2SO4 forms HNO2 which forms NO gas and HNO2
NO2- + 2H+ ---> HNO2
HNO2 + H2O + H+ ---> HNO3 + NO
For NO3-
NO3- oxidized Fe2+ to Fe3+ and gives off NO gas in presence of H2SO4
3Fe2+ + 4H+ + NO3- ---> 3Fe3+ + NO + 2H2O
the formation of brown ring confirms prescence of NO3- in original sample solution
[Fe(H2O)5]^2+ + NO ---> Fe(H2O)5NO]^2+ (brown ring)
2. For Cl- and Br-
Add AgNO3 solution,
AgNO3 + Cl- --> AgCl (white ppt) + NO3-
AgNO3 + Br- ---> AgBr (brown precipiate) + NO3-
So both Cl- and Br can be tested.
3. Ksp AgCl = 1.7 x 10^-10
Detection limit for [Cl-] = Ksp/[Ag+] = 1.7 x 10^-10/0.067 = 2.54 x 10^-9 M
Ksp AgBr = 4.1 x 10^-13
Detection limit for [Br-] = Ksp/[Ag+] = 4.1 x 10^-13/0.067 = 6.12 x 10^-12 M
Ksp Agl = 1.5 x 10^-16
Detection limit for [I-] = Ksp/[Ag+] = 1.5 x 10^-16/0.067 = 2.24 x 10^-15 M