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Questions and Answers
Which ion group utilizes 3M HCl as a precipitant to form a white precipitate?
Which ion group utilizes 3M HCl as a precipitant to form a white precipitate?
- Group I Cations (correct)
- Group III Cations
- Group II Cations
- Group IV Cations
What is the primary objective stated for the experiment?
What is the primary objective stated for the experiment?
- Determine the molecular weight of compounds.
- Establish the atomic number of elements.
- Identify chemical reactions using ion properties.
- Group the ions according to their solubility. (correct)
Which ions are members of the Group I Cations identified in the experiment?
Which ions are members of the Group I Cations identified in the experiment?
- Ba+2, Ca+2, Mg+2
- Hg2+, Na+, K+
- Pb+2, Hg22+, Ag+1 (correct)
- Cu+2, Ag+1, Pb+2
The grouping of elements can be based on their:
The grouping of elements can be based on their:
What is the result of treating the identified cations in Group I with a precipitating agent?
What is the result of treating the identified cations in Group I with a precipitating agent?
What is the effect of using a lower than 0.3M acid concentration in the solution when precipitating group II cations?
What is the effect of using a lower than 0.3M acid concentration in the solution when precipitating group II cations?
Which of the following sulfides forms a yellow precipitate when reacting in the Na2S soluble Sn subgroup?
Which of the following sulfides forms a yellow precipitate when reacting in the Na2S soluble Sn subgroup?
Which of the following metals does not dissolve when the acid concentration exceeds 0.3M?
Which of the following metals does not dissolve when the acid concentration exceeds 0.3M?
What color is the precipitate formed from CdS in the context of group II cations?
What color is the precipitate formed from CdS in the context of group II cations?
Which combination of cations from the Sn subgroup would produce a brown precipitate when Na2S is added?
Which combination of cations from the Sn subgroup would produce a brown precipitate when Na2S is added?
What happens after adding Na2S in the procedure for separating basic constituents?
What happens after adding Na2S in the procedure for separating basic constituents?
Which ions are present in the final centrifugate after adding ammonia and ammonium chloride?
Which ions are present in the final centrifugate after adding ammonia and ammonium chloride?
After the first step of separation which groups are precipitated?
After the first step of separation which groups are precipitated?
What is the primary ion that remains in the centrifugate after the second step of treatment with H2S?
What is the primary ion that remains in the centrifugate after the second step of treatment with H2S?
Which treatment step leads to the formation of Group 4 precipitates?
Which treatment step leads to the formation of Group 4 precipitates?
Which component of sodium sulfide reagent is responsible for oxidizing ions into higher oxidation states?
Which component of sodium sulfide reagent is responsible for oxidizing ions into higher oxidation states?
What is the reaction of HCl with FeS intended to generate?
What is the reaction of HCl with FeS intended to generate?
In the Group III Cations: Ammonium Sulfide Group, what does the precipitant (NH4)2S primarily form with Iron cations?
In the Group III Cations: Ammonium Sulfide Group, what does the precipitant (NH4)2S primarily form with Iron cations?
What is the typical result of mixing alkaline earth metals with the precipitant (NH4)2CO3?
What is the typical result of mixing alkaline earth metals with the precipitant (NH4)2CO3?
Which of the following ions is precipitated by NaOH in the Al Subgroup?
Which of the following ions is precipitated by NaOH in the Al Subgroup?
What is produced when thioacetamide is hydrolyzed in water?
What is produced when thioacetamide is hydrolyzed in water?
What is the color of the precipitate formed from Mn+2 in the Group III Cations?
What is the color of the precipitate formed from Mn+2 in the Group III Cations?
Which of the following components prevents the hydrolysis of sulfides in the sodium sulfide reagent?
Which of the following components prevents the hydrolysis of sulfides in the sodium sulfide reagent?
What is required for complete precipitation in the context of Group IV cations?
What is required for complete precipitation in the context of Group IV cations?
Which of the following cations is known to become very soluble in the presence of high concentrations of ammonium salt?
Which of the following cations is known to become very soluble in the presence of high concentrations of ammonium salt?
Which group do the members Li+, Na+, K+, and NH4+ belong to?
Which group do the members Li+, Na+, K+, and NH4+ belong to?
What happens when a solution containing Cu2+, Co2+, and other ions is treated with 3M HCl?
What happens when a solution containing Cu2+, Co2+, and other ions is treated with 3M HCl?
Which of these ions cannot be precipitated with the alkaline group?
Which of these ions cannot be precipitated with the alkaline group?
After adjusting the acid concentration to 0.3 M and adding H2S, which ion is expected to precipitate?
After adjusting the acid concentration to 0.3 M and adding H2S, which ion is expected to precipitate?
Which of the following steps is NOT involved in the process after centrifugation and adding (NH4)2S?
Which of the following steps is NOT involved in the process after centrifugation and adding (NH4)2S?
Which cation remains in solution along with alkali or soluble groups during analysis?
Which cation remains in solution along with alkali or soluble groups during analysis?
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Study Notes
Experiment Objectives
- The experiment uses solubility principles to group ions that form precipitates when reacting with specific reagents.
- Developing a systematic process for identifying ions is also a goal.
Fundamental Concepts
- Elements are classified based on their atomic numbers in the Periodic Table.
- Elements can also be categorized by the properties of their ions.
- Ions with similar characteristics are grouped together.
Group I Cations: Silver Group
- Members: Pb+2, Hg22+ or Hg+1, Ag+1
- Group Precipitant: 3M HCl
- Result: White precipitate of PbCl2, Hg2Cl2, AgCl
Group II Cations: Hydrogen Sulfide Group
- Members: Cu+2, Bi+3, Cd+2, Pb+2, Sn+2/+4, As+3/+5, Sb+3/+5, Hg+2
- Group Precipitant: Adjust H+ to 0.3M and saturate with H2S (using Thioacetamide)
- Cu Subgroup: CuS – Black ppt, Bi2S3 – Brown-Black ppt, CdS – Yellow ppt, PbS – Black ppt.
- Sn Subgroup: SnS – Brown ppt, SnS2 – Yellow ppt, As2S3 and As2S5 – Yellow ppt, Sb2S3 – Orange Red ppt, HgS – Black ppt, If Red (Cinnabar).
Group II Cations: Hydrogen Sulfide Group (Acid Concentration)
- The solution's acid concentration must be adjusted to 0.3M to precipitate Group II members.
- Below 0.3M, Group III sulfides may precipitate with Group II.
- Above 0.3M, some Group II sulfides like Bismuth, Cadmium, Lead, Tin, and Antimony may dissolve.
Group II Cations: Hydrogen Sulfide Group (Sodium Sulfide Reagent)
- Sodium sulfide reagent comprises various components:
- Na2S: Precipitates the sulfides of the ions.
- Na2S2: Oxidizes the ions to their higher oxidation states.
- NaOH: Prevents hydrolysis of the sulfides.
Group II Cations: Hydrogen Sulfide Group (Sources of Hydrogen Sulfide Gas)
- Sources of hydrogen sulfide gas (H2S):
- Reaction of HCl with FeS (Pyrite) in a Kipps Type Generator: FeS + 2HCl → FeCl2 + H2S↑
- Hydrolysis of Thioacetamide in slightly acidic or basic media at elevated temperature: CH3CSNH2 + H2O → CH3CONH2 + H2S↑
Group III Cations: Ammonium Sulfide Group
- Members: Fe+2/+3, Co+2/+3, Ni+2, Mn+2, Al+3, Cr+3, Zn+2
- Group Precipitant: (NH4)2S or Combination of NH4Cl, NH3, and H2S (insoluble), NaOH and Na2O2 (soluble).
- Fe Subgroup: FeS and Fe2S3 – Black ppt, CoS – Black ppt, NiS – Black ppt, MnS – Flesh or Salmon ppt
- Al Subgroup: Al(OH)3 – White gelatinous ppt, Cr(OH)3 – Grayish green ppt, ZnS – White ppt
Group IV Cations: Alkaline Earth Group
- Members: Mg+2, Ca+2, Sr+2, Ba+2
- Group Precipitant: (NH4)2CO3, NH3, NH4Cl, 95% EtOH mixture.
- Result: White ppt of: MgCO3, CaCO3, SrCO3, BaCO3.
- 95% Ethanol is needed for complete precipitation.
- MgCO3 is soluble in high ammonium salt concentrations.
- MgCO3 may not precipitate with the Alkaline Earth group and remain in solution with the alkali or soluble group.
Group V Cations: Alkali/Soluble Group
- Members: Li+, Na+, K+, Hypothetical Alkali Metal: NH4+
- Group Precipitant: None.
- Result: No precipitate forms.
Exercise: Identify the Group of Cations
- Mn +2, Ca +2, Na +2, Hg2+2, Cd +2, Sn +4, Zn +2
- Group I: Hg2+2
- Group II: Cd +2, Sn +4
- Group III: Mn +2, Zn +2
- Group IV: Ca +2
- Group V: Na +2
- Group II, III, IV: Group IV - Ca+2
Exercise: Stepwise Separation of Cations
- Solution containing: Cu+2, Co2+ , As3+ , K+ , Ag+, Zn2+ , Sr2+
- Step 1: Add 3M HCl
- Ppt: AgCl (white ppt)
- Centrifugate: Cu+2, Co2+ , As3+ , K+ , Zn2+ , Sr2+
- Step 2: Adjust Acid Concentration to 0.3M + H2S
- Ppt: CuS & As2S3
- Centrifugate: Co2+ , K+ , Zn2+ , Sr2+
- Step 3: Add Na2S to CuS & As2S3 Ppt
- Ppt: CuS
- Centrifugate: As3+
- Step 4: Add (NH4)2S to Co2+ , K+ , Zn2+ , Sr2+ Centrifugate
- Ppt: ZnS & CoS
- Centrifugate: Sr2+ K+
- Step 5: Add NaOH & Na2O2 to ZnS & CoS ppt
- Ppt: Co(OH)3
- Centrifugate: Zn2+
- Step 6: Add NH3, NH4Cl, (NH4)2CO3 & 95% :C2H5OH to Sr2+ K+ Centrifugate
- Ppt: SrCO3
- Centrifugate: K+
Separation of Basic Constituents Procedure
- 3-1: Mix 10 drops each of Ag+, Sb+3, Zn+2, Ba+2, Na+ and 1-2 drops of 3M HCl. Stir and centrifuge.
- Ppt: Group 1
- 3-2: Evaporate the centrifugate to almost dryness. Add 1 drop of 12M HCl and 39 drops of distilled water. Saturate with H2S by adding 10 drops of 10% thioacetamide. Heat in a water bath for 10 minutes.
- Ppt: Group 2
- 3-3: Make the centrifugate alkaline with NH4OH and 4 drops of NH4Cl. Saturate with H2S by adding 10 drops of 10% thioacetamide. Heat in a water bath for 10 minutes. Stir and centrifuge.
- Ppt: Group 3
- 3-4: Evaporate the centrifugate to half its volume. Add 3 drops of 6M NH4OH and heat in a water bath. Add 15 drops of 3M (NH4)2CO3 and 15 drops of 95% C2H5OH. Stir and centrifuge.
- Ppt: Group 4
- Centrifugate: Group 5
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