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"; Chemistry Pre Lab Questions-QUALITATIVE ANALYSIS OF UNKNOWN IONS IN A SOLTUION - Chem Homework Help


1) What questions will be answered with the results obtained from this experiment?

2) What kinds of selective precipitation will be used to detect the presence of metal cations? What kind of selective precipitation will be used to detect the presence of anions?

3) What is the main question this experiments seeks to answer?

4) What is the best way to insure that there are no false positives or that there is not contamination or interference from unwanted ions in the solution?

5) How is selective precipitation specifically used to identify unknown ions in this experiment?

6) What technique is used to separate precipitate from the ions that remain dissolved in the solution? What techniques are used to treat the precipitates for additional analyses?

7) Discuss the importance of selective precipitation.

8) What results will be collected and recorded in your lab notebook?

9) What can be expected if the ions are correctly identified? What are some things that might prevent the ions from being correctly identified?

10) What is this experiment designed to measure? What will the results NOT tell us about the experiment?

11) What special safety precautions do you need to be aware of for this experiment?



In a vacated company laboratory, you find an unlabeled bottle of aqueous solution. It is known that the cation(s) is(are) from the following: Ag+, Hg22+, Pb2+, Cu2+, Bi3+, As3+, Co2+, Ni2+, Fe3+, and Al3+; and the anions from SO42-, CO32-, PO43-, Cl, and NO3. Design an experiment to identify what ion(s) is(are) present in this unlabeled solution. 

  • Design a detailed procedure to separate the ions into groups
  • Identify the ions in the unknown solution using the information in the Background Section.
  • Develop a flow chart to clearly show the steps, and add individual tests to the flow chart to show the identification. List the techniques and equipment you would need to use.


The first step in qualitative analysis is to separate the ions into groups, because many ions may give similar results when the same reagents are added, and hence may interfere with each other. If present, Ag+, Hg22+ and Pb2+ will all form white precipitate when HCl is added, thus separate from the rest of the ions in the solution. After Ag+, Hg22+, Pb2+ are removed from the solution, Cu2+, Bi3+, As3+, if present, will all form precipitate with H2S in acidic conditions, thus separate from the rest of the solution. Co2+, Ni2+, Fe3+, and Al3+, if present, will precipitate when NaOH is added, thus separate from the rest of the solution.

After separation of the ions, individual tests are to be performed to identify the ions. For example, a white precipitate forms when HCl is added to the unknown solution, indicating one, two, or all three cations from Group I (Ag+, Hg22+, Pb2+) may exist. After adding excess HCl to ensure that all of the Group I cations have precipitated out of the solution, the precipitate is collected (using which technique?), washed (using which technique?) and dried. Always keep the rest of the clear solution because you need to analyze it for other ions later! The precipitate collected is then analyzed for existence of Ag+, Hg22+ or Pb2+:

PbCl2 is soluble in hot water while AgCl and Hg2Cl2 are not. Therefore adding water the precipitate and heating the mixture will dissolve PbCl2 and separate it from AgCl and Hg2Cl2 (using which techniques?). AgCl will redissolve if NH3 is added,

AgCl(s) + 2 NH3 (aq) ⇌ Ag(NH3)2+(aq) + Cl(aq)

but will form a white precipitate again if the solution is acidified with HNO3,

Ag(NH3)2+(aq) + Cl(aq) + 2H+(aq) AgCl(s) (white) + 2 NH4+(aq)

This series confirms the existence of Ag+.

Hg22+ is identified by the following reaction,

Hg2Cl2(s) + 2 NH3(aq)Hg(s) (black) + HgNH2Cl(s) (white) + NH4+(aq) + Cl(aq)

Pb2+ is confirmed by the K2CrO4 test:

Pb2+(aq) + CrO42-(aq)PbCrO4(s) (yellow)

After Group I cations are removed from the unknown solution, Group II cations are precipitated by adding 6M HCl and thioacetamide (CH3CSNH2) which hydrolyzes to give H2S:

CH3CSNH2(aq) + 2H2O(l) + H+(aq) ⇌ CH3COOH(aq) + NH4+(aq) + H2S(aq)

H2S will then precipitate Cu2+, Bi3+, As3+ from the solution, if they are present. Copper and bismuth sulfides are insoluble in basic thioacetamide while arsenic sulfide dissolves in the form of thio complex ion, AsS33-, thus separating arsenic from copper and bismuth.

Arsenic is tested as follows,

As2S3(s) + H2S (aq) + NaOH (aq) → AsS33-(aq)

AsS33-(aq) + HCl(aq) → As2S3(s)

As2S3(s) + H2O2 (basic) → AsO43-(aq)

AsO43-(aq) + Mg(OH)2 (magnesia mixture) → MgNH4AsO4 (s) (white)

CuS and Bi2S3 dissolve in hot HNO3 forming Cu2+ and Bi3+ ions; addition of NH3 will form a white precipitate, Bi(OH)3, with Bi3+, and a deep blue color complex solution, Cu(NH3)42+, with Cu2+.

Co2+, Ni2+, Fe3+, and Al3+ all form insoluble hydroxides when NaOH is added, but Al(OH)3 will re-dissolve if excess NaOH is added because of its amphotericity,

Al(OH)3(s) + OH(aq) ⇌ Al(OH)4(aq)

Therefore, Co(OH)2(s, lavender), Ni(OH)2(s, green), Fe(OH)3(s, reddish-brown) are separated from Al(OH)4(aq). They will dissolve again if the pH is adjusted to acidic. Since the tests for these three ions, Co2+, Ni2+, Fe3+, do not interfere significantly with each other, the solution is simply divided into three portions for individual tests:

Ni2+(aq) + 6NH3(aq) ⇌ Ni(NH3)62+(aq)

Ni(NH3)62+(aq) + 2HDMG ⇌ Ni(DMG)2(s) (red) + 2NH4+(aq) + 4NH3(aq)

where HDMG is demethylglyoxime, (CH3)2C2(NOH)2.

Co2+ is tested by first adding NaF to complex with Fe3+ (FeF63-) to prevent interference from Fe3+, followed by addition of saturated NH4SCN in ethanol solution:

Co2+ + 4SCNCo(SCN)42-(blue)

Fe3+ is tested by adding KSCN:

Fe3+(aq) + SCN(aq)FeSCN2+(aq) (red)

Aluminum is tested by the following steps:

  1. Al(OH)4(aq) + 16M HNO3 → Al3+(aq)
  2. Al3+(aq) + 15M NH3 → Al(OH)3(s)
  3. Al(OH)3(s) + 3M CH3COOH → Al3+(aq)
  4. Al3+(aq) + aluminum reagent + 6M NH3red precipitate (Al(OH)3-Al)

The anions are analyzed in a similar fashion. The BaCl2 group where addition of BaCl2 forms white precipitates with SO42-, CO32-, PO43-,

SO42-(aq) + Ba2+(aq) ⇌ BaSO4(s)

CO32-(aq) + Ba2+(aq) ⇌ BaCO3(s)

2PO43-(aq) + 3Ba2+(aq) ⇌ Ba3(PO4)2(s)

The difference is BaSO4(s) will not dissolve in HCl , while both BaCO3(s) and Ba3(PO4)2(s) do dissolve in HCl; BaCO3(s) also effervesce with release of CO2.

BaCO3(s) + 2H+(aq) ⇌ Ba2+(aq) + CO2(g) + H2O(l)

The AgNO3 group, where addition of AgNO3 precipitates CO32-, PO43-, and Cl. Ag2SO4 is slightly soluble.

CO32-(aq) + 2Ag+(aq) ⇌ Ag2CO3(s) (white)

PO43-(aq) + 3Ag+(aq) ⇌ Ag3PO4(s) (yellow)

Cl(aq) + Ag+(aq) ⇌ AgCl(s) (white)

Addition of HNO3 causes Ag2CO3 and Ag3PO4 to dissolve, with Ag2CO3 also causing effervescence, while AgCl remains as a white precipitate.

The soluble group, NO3, is tested by the brown ring test. The addition of H2SO4 and FeSO4 causes a brown ring of Fe(H2O)5NO2+ to appear at the interface of the two layers.

NO3(aq) + 4H+(aq) +4Fe2+(aq) + 3H2O → 3Fe3+(aq) + Fe(H2O)5NO2+(aq)

Identification of the Silver group ions: Ag+, Hg22+, and Pb2+

These ions are removed from the solution by adding excess 6M HCl to the solution and stirring with a stirring rod. After centrifugation, the precipitate will stick to the wall and bottom of the test tube and the supernatant solution will be clear. Test for complete precipitation by adding 2 more drops of 6M HCl to the clear supernatant liquid. If the solution turns cloudy, add two more drops of HCl, stir and centrifuge. Repeat until the supernatant liquid is clear when HCl is added.

Decant the supernatant solution and wash the precipitate with some cold water, centrifuge and decant the wash solution. The silver group ions, if any, are now in the precipitate.

Add some distilled water to the precipitate and place the test tube in a hot water bath, stir and heat for a few minutes. Quickly centrifuge and decant the solution in a clean test tube labeled “Pb2+”. Repeat the above step two more times and combine the solutions after centrifuging in the labeled test tube. The remaining precipitate, if any, will be analyzed for Ag+ and Hg22+.

Test for Ag+ and Hg22+: Add 20 drops of 4M NH3 to the precipitate. If a grey to black precipitate forms Hg22+ is present. Centrifuge and decant the clear liquid in a clean test tube. Acidify the solution with 4M HNO3. A white precipitate of AgCl or cloudiness confirms the presence of Ag+.

Test for Pb2+: Pour half of the solution from the Pb2+-labeled test tube into a clean test tube, add a few drops of K2CrO4 and observe; add a few drops of KI to the other portion of the solution, observe.

Identification of the Copper-Arsenic group ions: Cu2+, Bi3+, and As3+

To the clear supernatant solution after removing the silver group ions, add a few drops of 6M HCl and 20 drops of thioacetamide (CH3CSNH2) and heat in a boiling water bath for 5 minutes. Add 1 mL of H2O and heat for another 10 min. Centrifuge and decant the supernatant into a clean test tube labeled “Group II”. Group II ions (Cu2+, Bi3+, and As3+), if present, will be in the precipitate.

Wash the precipitate twice with 10 drops of 0.1M HCl, centrifuge and decant. Discard the wash solutions.

Add 15 drops of 4M NaOH and 4 drops of thioacetamide to the precipitate. Heat in a water bath for 5 min. Centrifuge and decant. Label the solution “As”. Analyze the precipitate which could be  either CuS or Bi2S3 or both.

Wash the precipitate with a mixture of 1 mL of water and 2 drops of NH4NO3, centrifuge and discard the wash solution. Add 15 drops of 6M HNO3 to the precipitate and heat in a water bath for 5 min. Stir constantly while heating. Note that free sulfur is formed. Centrifuge and discard the sulfur. Analyze the solution for copper and bismuth ions.

Test for Cu2+ and Bi3+: Make the solution basic with 15M aqueous NH3. A deep blue solution (depending on the copper ion concentration) due to complex ion, Cu(NH3)42+, proves the presence of copper ions. A white precipitate of Bi(OH)3 forms if bismuth ions are present. Centrifuge and discard the supernatant liquid. Wash the precipitate with 10 drops of hot water, centrifuge and discard the wash solutions. Add 3 drops of 6M NaOH and 2 drops of freshly prepared SnCl2 solution. A black precipitate of Bi proves the presence of bismuth ions.

Test for As3+: Acidify the As-labeled solution with 1M HCl. Heat in a water bath for several minutes. Centrifuge and discard the solution. A yellow residue could be As2S3. Add 12 drops of 4M NH3 and 6 drops of 3% H2O2 to the residue. Heat in a water bath for several min. Centrifuge and discard any residue which could be sulfur. Add 2 drops of 15M NH3 and 5 drops of magnesia mixture to the centrifuge. A white precipitate of MgNH4AsO4, which forms slowly, proves the presence of arsenic ions.

Identification of the Nickel-Aluminum group ions: Co2+, Ni2+, Fe3+ and Al3+

Get 1 mL of the clear supernatant solution labeled with “Group III” from removing Group II ions and place in a clean test tube. Add 2 mL of 6M NaOH and mix thoroughly. The color of the precipitate could give an indication of which ions are present. Add a few drops of distilled water and heat in a boiling water bath for 2 min. Cool and centrifuge. Decant the supernatant liquid which may contain Al(OH)4 into a clean test tube and label with “Al”. Wash the precipitate twice with 4 mL portions of distilled water, centrifuge after each washing and discard the wash solution.

Add 1 mL 6M H2SO4 to the washed precipitate , mix and heat in a boiling water bath for several min. Remove the test tube and dilute the solution to 3 mL using distilled water. Cool and divide the solution into 3 equal portions in 3 clean test tubes.

Test for Ni2+: To the first portion, add 15M NH3 dropwise until basic. Centrifuge if a precipitate forms. Do not decant. Add 5 drops of HDMG to the supernatant solution. A cherry red precipitate of Hi(DMG)2 proves the presence of nickel ions.

Test for Co2+: To the second portion, add a small amount of solid NaF, stir. Add additional NaF with stirring until some of the solid remains at the bottom of the test tube. Add several crystals of NH4SCN. A blue complex of Co(SCN)42- proves the presence of cobalt ions.

Test for Fe3+: To the third portion, add a few drops of KSCN solution. A blood red color proves the presence of iron ions.

Test for Al3+: Slightly acidify the supernatant solution labeled with “Al” with 16M HNO3. Add 15M NH3 until the solution is basic. Stir the solution thoroughly. Centrifuge and decant. If aluminum is present a white gelatinous precipitate will be present. Wash the precipitate three times with 10 drops of hot water, centrifuge and decant after each wash. Discard the wash solutions. Add 5 drops of 3M CH3COOH to the precipitate. Stir to dissolve the precipitate, centrifuge and decant in a clean test tube. Discard any undissolved precipitate. Add 2 drops of aluminum reagent. Stir, then make slightly basic with 6M NH3. Mix and centrifuge. A red precipitate confirms the presence of aluminum ions.

Identification of NO3

To 1 mL of the sample solution, add 3 mL of concentrated H2SO4 and mix. Let the test tube cool to room temperature. Hold the test tube in an incline position and pour down the inner side of the test tube several mL of FeSO4 solution so that there will be two distinct layers. The formation of a brown ring at the interface of the two liquids indicates the presence of the nitrate ions.

Analysis of an Unknown

Individual Pre-lab

  1. The main goal/purpose of the experiment is (what are you trying to discover in this lab):
    1. What  are the questions we need to answer with the results obtained from this experiment?
  2. b. The hypothesis(es) we seek to test in this experiment is(are) (what is the basis of your experiment, see Ch.1 in your text):
      1. What tests will be used to test for the presence of unknowns cations?
    1. What tests will be used to test for the  presence of unknown anions?
  3. b. The key question the experiment seeks to answer is (what problem are you addressing?)

(see 1 a):

  1. c. The controls involved in this experiment are (a control is used to minimize the unintended influence of other variables on the same system)

3. What is the best way to insure that there are no false positives or that there is not contamination  or interference.?

4. The key concept(s) or theory(ies) behind the experiment is(are) (define terms like pure substance, mixture, sublimation, etc. can you explain the relevant theory):

  1. How can selective precipitation be used to identify unknown ions?

b. The important technique(s) used in this experiment is(are) (sublimation, etc.)  Please describe:

  1. What technique is used to separate precipitate from ions that remain dissolved in the solution?
  2. What technique is used to treat precipitate or solution for presence of an unknown ion?
  3. The experiment is based on the following assumptions (identify your assumptions and determine if they are justifiable):
  4. Discuss the importance of selective participation.  
  5. The data that will be collected in the experiment are (what do you plan to record in your notebook)
    1. What results will be collected and recorded in your lab notebook?
    2. The potential consequences of the experiment are:
    3. What can be expected if positive results are obtained?
    4. What might prevent positive results from being obtained?
  6. The point of view behind the experiment is (make sure it’s scientific).
  7. What is this experiment designed to measure?
  8. What can the results NOT tell us about the experiment?
  9. What special safety precautions do you need to be aware of for this experiment?


Group Experimental Design Pre-lab

Include a detailed procedure along with a flowchart drawn in PowerPoint to describe how your separation and tests for unknown ions will be conducted.   This procedure should be written stepwise with the initials of the student who is performing that step in the procedure.  Here is an example:   

  1. Weigh substance- AW (these are the initials of student performing task)
  2. Get the volume of substance – EE
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