You have three mixtures consisting of powdered inorganic solids on your desk. They could contain the — Organic Chemistry Chemistry Question
PREPARATORY PRACTICAL PROBLEM P2
You have three mixtures consisting of powdered inorganic solids on your desk.
They could contain the following compounds:
a) (NH4)2CO3, AgNO3, BaCl2·2 H2O, NH4NO3, NiCl2·2 H2O
b) ZnO, KI, Pb(NO3)2, BaSO4, MnO2, Mg
c) CaCO3, NH4I, FeSO4·7 H2O, TiO2, CuCl2·2 H2O
You can use distilled water, HCl, HNO3, NH3, and NaOH solutions, all with the concentration of 2 mol dm –3 and, moreover, pH paper, test tubes and a Bunsen burner.
(Not all of the compounds listed are present in the unknown samples.)
Determine which compounds are present in the mixture and which are not.
Model Answer
Example of one of the solutions:
The following compositions were handed out:
a) AgNO3, BaCl2·2 H2O, NH4NO3 absent: (NH4)2CO3, NiCl2·2 H2O
Colorless no nickel.
No gas evolution observed with acids no carbonate.
Sample does not dissolve in water the precipitate must be AgCl. Indeed, the solution clears up on adding ammonia.
Heating the solution with NaOH produces ammonia, as seen on the indicator paper at the mouth of the test tube. (Or smelt.)
b) ZnO, Pb(NO3)2, Mg absent: BaSO4, KI, MnO2
Grey powder consisting of a dark and white component contains Mg or MnO2.
After adding water a grey, heterogeneous substance remains ZnO or BaSO4 might also be present.
Slowly dissolves without residue in nitric acid with gas evolution Mg and ZnO is present.
A white precipitate remains in HCl Pb(NO3)2 is present. KI can be excluded as yellow PbI2 was not seen.
c) CaCO3, CuCl2·2 H2O absent: NH4I, FeSO4·7 H2O, TiO2
Sample dissolves completely in acids with gas evolution TiO2 absent, CaCO3 present. FeSO4 must also be absent because CaSO4 is not formed.
The green powder dissolves in acid leaving a bluish solution. Ammonia gives a dark blue discoloration CuCl2
NH4I cannot be present together with Cu2+ as iodine would be produced.
Note your experimental findings in detail. Explain every conclusion (positive or negative).
Model Answer
Detailed findings and explanations from the solution key:
- Mixture a): Colorless solution indicates Ni2+ (and NiCl2·2 H2O) is absent. No gas evolution with acids indicates carbonate (and (NH4)2CO3) is absent. Undissolved sample in water indicates precipitate formation (AgCl from AgNO3 and BaCl2·2 H2O), which clears up upon adding ammonia due to complexation. Heating the solution with NaOH produces ammonia (detected by indicator paper or odor), confirming the presence of NH4+ (from NH4NO3).
- Mixture b): Grey powder contains Mg. Complete dissolution in nitric acid with gas evolution indicates Mg and ZnO are present, while insoluble MnO2 and BaSO4 are absent. White precipitate remaining in HCl indicates Pb2+ (from Pb(NO3)2). Absence of yellow PbI2 precipitate excludes KI.
- Mixture c): Complete dissolution in acids with gas evolution indicates CaCO3 is present and TiO2 is absent. Complete dissolution also means FeSO4 is absent (as CaSO4 would have precipitated). The green powder dissolves in acid to yield a bluish solution that turns dark blue with ammonia, confirming CuCl2. NH4I is absent because Cu2+ would have oxidized iodide to form visible iodine.
Include reaction equations where possible.
Model Answer
Reaction equations included in the solution key:
- Mixture a):
Ag+ + Cl– = AgCl
AgCl + 2 NH3 = Ag(NH3)2+ + Cl–
NH4+ + OH– = NH3↑ + H2O
- Mixture b):
Mg + 2 H+ = Mg2+ + H2↑
Pb2+ + 2 Cl- = PbCl2
- Mixture c):
Cu2+ + 4 NH3 = Cu(NH3)42+