You have been provided with eight vials, each of which is labeled with a number from 1 to 8. Each vi — Qualitative Analysis Chemistry Question
Problem Context
You have been provided with eight vials, each of which is labeled with a number from 1 to 8. Each vial contains one of the following chemicals:
Na3PO4, NH4Cl, ZnCl2, KNO3, Mg(OH)2, Pb(NO3)2, CaCO3, Na2SO3.
You are allowed to use distilled water, test tubes or well plates, and only TWO additional reagents from the following choices:
6 M H2SO4, 6 M HCl, 6 M AgNO3, phenolphthalein indicator solution
You must designate your choice of reagents prior to the start of your testing.
Give a brief description of your experimental plan.
Model Answer
A good plan involved stating that samples of each of the eight unknowns would be placed in wells of the spot plates, distilled water would be added and observations would be made regarding the solubility of the salts. Then, each of the SELECTED reagents would be added and specific identifying tests would be detailed. It should be noted that several combinations of reagents could have been used to identify the unknowns.
For example, a good test might include these tests.
1. Place each sample into each of three wells in the spot plate. Add distilled water to each and record solubility results.
2. Two of the samples will not dissolve. Add HCl to these. One sample should dissolve and the other should fizz. The former is magnesium hydroxide, the latter is calcium carbonate.
3. Add HCl to the other solutions. Lead (II) nitrate will form a precipitate. Sodium sulfite will release sulfur dioxide which can be identified by odor.
4. Add silver nitrate to the other four solutions. The sample which does not react is potassium nitrate. The yellow precipitate is produced by sodium phosphate and the two white solids are zinc chloride and ammonium chloride.
5. Prepare additional solutions of the two chlorides and the identified sodium phosphate. Add the latter to the chlorides. The zinc chloride will form a precipitate while the ammonium chloride will not react.
An average plan did not detail the expected results of specific tests.
A weak plan did not include solution formation or specific results of tests.
Record your data and other observations.
Model Answer
An example of good work on observations and results would be:
Reagent | Sample 1 | Sample 2 | Sample 3 | Sample 4 | Sample 5 | Sample 6 | Sample 7 | Sample 8
water | soluble | insoluble | soluble | insoluble | soluble | soluble | soluble | soluble
HCl | No Rx | dissolves | White ppt | fizz | No Rx | Sharp odor | No Rx | No Rx
AgNO3 | White ppt | | | | No Rx | | Yellow Ppt | White ppt
Na3PO4 | No Rx | | | | | | | White ppt
Identify the substance in each numbered vial.
Vial # Contains
1.
2.
3.
4.
5.
6.
7.
8.
Model Answer
The order should be as follows:
Vial 1: NH4Cl
Vial 2: CaCO3
Vial 3: Pb(NO3)2
Vial 4: Mg(OH)2
Vial 5: KNO3
Vial 6: Na2SO3
Vial 7: Na3PO4
Vial 8: ZnCl2
Explain clearly how you arrived at the identity of each of the vial’s contents.