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EXPERIMENT 14 Properties of Lead(11), Silver, and Mercury(1) lons MATERIALS AND EQUIPMENT Solutions: concentrated (15 M) ammonium hydroxide (NH,OH), dilute (6 M) hydrochloric acid (HCI), 0.1 M lead(II) nitrate [Pb(NO3)2], 0.1 M mercury(I) nitrate (Hg2(NO3)2), dilute (6 M) nitric acid (HNO3), 0.1 M potassium chromate (K,CrO2), and 0.1 M silver nitrate (AgNO3); “known" solution containing Pb2+, Ag+, and Hg2+ ions; and "unknown” solutions to be ana- lyzed. DISCUSSION The object of this experiment is to investigate some of the chemical properties of lead(II), sil- ver, and mercury(I) cations and to identify these ions in solution. In the broader scheme of qualitative analysis these three cations are known as the silver group. They are grouped to- gether because of the common property of forming water-insoluble chlorides, a property that enables then to be separated from most other cations. First we will run some selected chemical reactions for each of these ions. Then a scheme of analysis based on these reactions will be used to separate and identify these cations in a "known” solution containing all three ions and in an "unknown” solution containing one or more of the ions. If necessary, see Experiment 13 for an explanation of un-ionized, total ionic, and net ionic equations. The net ionic equations representing the reactions to be observed are: Lead(II) (Pb(NO3)2 solution] Pb2+(aq) + 2 Cl-(aq) —+ → PbCl(s) (white ppt forms) PbCl2(s) hot H20 → Pb2+ (aq) + 2C1-(aq) (ppt dissolves) Pb2+(aq) + CrO22-(aq) + PbCrO4(s) (bright yellow ppt forms) Silver (AgNO3 solution] (white ppt forms) Ag+(aq) + Cl-(aq) - AgCl(s) AgCl(s) + 2 NH,OH(aq) - Ag(NH3)2+(aq) + Cl-(aq) + 2 H2O(1) - (l(ppt dissolves) (white ppt forms) Ag(NH3)2+(aq) + Cl-(aq) + 2 H+(aq) → AgCl(s) + 2 NH4+(aq) - 117 - - Mercury(I) [Hg(NO3)2 solution] Hg22+ + 2 C1- -→ Hg2C1,(s) (white ppt forms) Hg2Cl2 + 2 NH,OH —— Hg(1) + HgNH,Cl(s) + 2 H2O + NH,+ + C1- (black ppt forms) PROCEDURE Wear protective glasses. General Instructions 1. Use distilled water throughout this experiment. 2. Since hot water is frequently needed in the procedure, fill a 400 ml beaker half full of water and start heating it to boiling before beginning to work with your cation solutions. 3. One mL quantities are used frequently throughout the procedure. You can save consider- able time by determining how many drops are needed to deliver 1 mL from your medicine drop- per or pipet and using this number of drops whenever 1 mL of reagent is required. 4. Submit a clean, labeled test tube to your instructor for your unknown solution. 5. Precipitates are washed to remove soluble ions. Washing is accomplished by adding the specified amount of solvent (usually water), agitating the mixture by gently shaking back and forth, and pouring off (decanting) the washing solvent. Shaking back and forth, rather than up and down, prevents the accumulation of large amounts of precipitate on the walls of the tube. 6. Immediately after making your observations, record them on the report form. WASTE DISPOSE OF PROPERLY Several toxic heavy metals and their solutions are used in this experiment. Dispose of all solutions and precipitates of Pb, Ag, Hg, and chromate compounds in the "heavy metals” waste bottle. This includes liquids decanted off a precipate during washing. symbol will be used to remind you to dispose of these wastes properly. AWAIT DISPOSE OF PROPERLY A. Tests for Lead(II) Ion 1. To 2 mL (no more) of 0.1 M lead(II) nitrate solution in a test tube add 1 mL of dil. (6 M) hydrochloric acid. WASTE DISPOSE OF PROPERLY 2. Allow the precipitate to settle and then separate it from the liquid by decanting the liquid. (Decanting means to pour the liquid off carefully, leaving the solid behind.) A loss of a small amount of precipitate at this point has no effect on the experimental results. precipitate settles. 3. Wash the precipitate with 2 mL of cold water, again decanting the liquid after the WASTE DISPOSE OF PROPERLY 4. Add 5 mL of water to the precipitate and place the tube in the beaker of boiling water. Heat the mixture for about 2 minutes, shaking frequently. All the precipitate should dissolve. - 118 - WASTE DISPOSE OF PROPERLY 5. Remove the tube from the beaker and add a few drops of 0.1 M potassium chro mate solution. A yellow precipitate of lead(II) chromate confirms the presence of lead(II) ions. B. Tests for Silver Ion 1. To 2 mL of 0.1 M silver nitrate solution in a test tube add 1 mL of dilute (6 M) HCI. WASTE DISPOSE OF PROPERLY 2. Allow the precipitate to settle and then decant the liquid. WASTE DISPOSE OF PROPERLY 3. Wash the precipitate with 2 mL of cold water, again decanting the liquid after the precipitate settles. 4. Add concentrated ammonium hydroxide dropwise to the precipitate until it all dissolves. (This should take less than 1 mL.) WASTE DISPOSE OF PROPERLY 5. Now add dil. (6 M) nitric acid dropwise until the solution is acidic (test with blue litmus paper). A white precipitate of silver chloride confirms the presence of silver ions. C. Tests for Mercury(I) Ion 1. To 2 mL of 0.1 M mercury(I) nitrate solution in a test tube add 1 mL of dilute (6 M) HCL. WASTE DISPOSE OF OPERLY 2. Allow the precipitate to settle and then decant the liquid. WASTE 3. Wash the precipitate with 2 mL of cold water, again decanting the liquid after the precipitate settles. DISPOSE OF PROPERLY WASTA DISPOSE OF PROPERLY 4. Add about 1 mL of concentrated (15 M) ammonium hydroxide to the precipitate. The formation of a black precipitate confirms the presence of mercury(I) ions. The precipitate is composed of finely divided black mercury (Hg) and white mercury(II) amido chloride (HgNH,Cl) and appears black overall. D. Analysis of a Known and an Unknown Solution The sequence of steps that follows is to be performed on a known solution containing all three silver group cations and on an unknown solution obtained from your instructor. With the known solution you should see evidence of all reactions described for each cation. The unknown may contain one, two, or three cations; you will not see any evidence of reaction for a cation that is not present. Step 4 of the analysis requires the use of a funnel for each solution tested. If you have two funnels, you can run the known and unknown solutions simultaneously. If you have only one funnel, run the known solution first. A flow chart of the silver group analysis is shown in Figure 14.1. 1. To 2 mL of the sample in a test tube add 1 mL of dil. (6 M) HCI. 2. Mix by shaking, allow the precipitate to settle, and decant the liquid. WASTE DISTODE OF 3. Wash the precipitate with 2 mL of cold water and decant the liquid. Wash the pre- cipitate again with 2 mL of cold water and decant the liquid. WASTE OLOPOSE OF OPERLY ional for cone, NAME SECTION DATE REPORT FOR EXPERIMENT 14 INSTRUCTOR Properties of Lead(11), Silver and Mercury(1) lons A. Test for Lead(II) Ion 1. Record your observations for (a) Part A.1. (b) Part A.4. (c) Part A.5. 2. Write the name, formula, and color of the precipitate formed in Part A.1. 3. Write the name, formula, and color of the precipitate formed in the confirmatory test for lead(II) ion (Part A.5). 4. What is accomplished by washing a precipitate? - 121 REPORT FOR EXPERIMENT 14 (continued) NAME B. Test for Silver Ion 1. Record your observations for (a) Part B.1. (b) Part B.4. (c) Part B.5. 2. Write the name, formula, and color of the precipitate formed in Part B.1. 3. Write the name, formula, and color of the precipitate formed in the confirmatory test for silver ion (Part B.5.). C. Tests for Mercury(I) Ion 1. Record your observations for (a) Part C.1. (b) Part C.4. - 122 -
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Surname 1
Student’s Name
Instructor
Course
Date
Properties of Lead (II), Silver and Mercury (I) Ions
Introduction
The chemical property of lead (II), silver and mercury (I) cations to forming water-insoluble
chlorides can be used for qualitative analysis. Notably, the three cations are referred to as the
silver group as the aforementioned chemical property enables them to be separated from most of
the other cations. In a qualitative analysis experiment of the mentioned cations, selected
chemical reactions for each of the ions are performed. A scheme of analysis based on the
reactions is then used to separate and identify the cations in a ‘known’ solution containing all
three ions and in an ‘unknown’ solution containing one or more of the ions.
Net ionic equations are followed during the reactions and can be used to explain what is
observed during qualitative analysis. The equations are:
Lead (II) [Pb(NO3)2 solution]
Pb2+ (aq) + 2Cl- (aq) → PbCl2(s) (white precipitate forms)
Heat
PbCl2 (s) →

Pb2+ (aq) + 2Cl- (aq) (ppt dissolves)

H2O
Pb2+(aq) + CrO42-(aq) → PbCrO4(s) (bright yellow solid)
Silver [AgNO3 solution]
Ag+(aq) + Cl-(aq) → AgCl(s) (white ppt forms)

Surname 2
AgCl(s) + 2NH4OH (aq) → Ag(NH3)2+ (aq) + Cl-(aq) + 2 H2O (l) (ppt dissolves)
Ag(NH3)2+ (aq) + Cl-(aq) + 2H+(aq) → AgCl(s) + 2NH4+ (white ppt forms)
Mercury [Hg2(NO3)2 solution]
Hg22+ (aq) + 2Cl- (aq) → Hg2Cl2(s) (white ppt forms)
Hg2+(aq) + NH3(aq) + Cl–(aq) + H2O(liq) → HgClNH2(s) + H3O+(aq) (black ppt
forms)
Materials and Equipment
i.

Concentrated (15 M) Ammonium Hydroxide (NH4OH)

ii.

Dilute (6 M) Hydrochloric acid (HCl)

iii.

0.1 M Lead (II) Nitrate [Pb(NO3)2]

iv.

0.1 M Mercury (I) Nitrate [Hg2(NO3)2]

v.

Dilute Nitric acid (HNO3)

vi.

0.1 M Potassium Chromate (K2CrO4)

vii.

O...


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