Group III Cations: Iron-Aluminum Qualitative Analysis

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Questions and Answers

Why is $NH_4Cl$ added during the precipitation of group III cations as hydroxides?

To decrease the concentration of $OH^-$ ions, preventing the precipitation of group IV sulfides.
To increase the concentration of $OH^-$ ions, ensuring complete precipitation of all group III hydroxides.
To increase the solubility of $Fe(OH)_3$, ensuring that it remains in solution.
To buffer the solution and maintain an optimal $pH$ for the selective precipitation of group III hydroxides. (correct)

In the context of selective precipitation using $H_2S$, what is the purpose of ensuring $Fe^{3+}$ is oxidized to $Fe^{2+}$?

To prevent the precipitation of iron as $Fe(OH)_3$, which has a lower $K_{sp}$ than $Fe(OH)_2$. (correct)
To ensure iron precipitates as a distinct compound, separate from other metal sulfides.
To facilitate the precipitation of $FeS$ with a higher $K_{sp}$ than other sulfides.
To increase the solubility of iron sulfides, preventing their precipitation.

What condition ensures the precipitation of $Fe^{3+}$ as $Fe(OH)_3$ during cation separation?

Maintaining a strongly acidic environment to prevent hydroxide formation.
Ensuring a high concentration of ammonium ions to promote complex formation.
Adding excess sulfide ions to form iron sulfide precipitates.
Adjusting the $pH$ to favor hydroxide formation while avoiding sulfide precipitation. (correct)

Why do sulfides like $MnS$, $ZnS$, $CoS$, and $NiS$ precipitate under specific conditions involving $NH_4Cl$, $NH_4OH$, and $H_2S$?

These conditions provide a controlled concentration of sulfide ions, exceeding the solubility product of these sulfides. (A) Signup and view all the answers

What is the visual indication that $Fe^{3+}$ has precipitated as $Fe(OH)_3$?

Formation of a reddish-brown precipitate. (C) Signup and view all the answers

What is the primary role of $NH_4Cl$ in the qualitative analysis of Group III cations?

To control $OH^-$ concentration, preventing the precipitation of Group IIIB cations and $Mg(OH)_2$. (C) Signup and view all the answers

Why is boiling a solution containing $NH_4Cl$ performed after the precipitation of Group II cations?

To remove $H_2S$ and prevent interference with the precipitation of Group IIIB cations. (A) Signup and view all the answers

What is the purpose of adding $NaOH$ and $H_2O_2$ to separate $Fe^{3+}$, $Al^{3+}$, and $Cr^{3+}$?

To form soluble aluminate ($AlO_2^−$) and oxidize $Cr^{3+}$ to chromate ($CrO_4^{2−}$). (D) Signup and view all the answers

In the separation of Group IIIA cations, what happens to $Cr^{3+}$ when $NaOH$ and $H_2O_2$ are added?

It is oxidized to chromate ($CrO_4^{2−}$), which appears yellow. (B) Signup and view all the answers

Which of the following is NOT a function of $NH_4Cl$ in the analysis of Group III cations?

Oxidizing $Mn^{2+}$. (B) Signup and view all the answers

What is the key difference in group reagents between Group III-A and Group III-B cations?

Group III-B uses $H_2S$ in addition to $NH_4Cl$ and $NH_4OH$. (D) Signup and view all the answers

What is the purpose of heating the solution after adding $NH_4Cl$?

To coagulate the precipitate, break $Cr$ amine complexes, and prevent $Mn^{2+}$ oxidation. (A) Signup and view all the answers

Why is it important to prevent the oxidation of $Mn^{2+}$ during the analysis of Group III cations?

Oxidation of $Mn^{2+}$ results in the precipitation of manganese oxides, complicating the separation. (C) Signup and view all the answers

What might happen if the boiling step after Group II cation removal is skipped?

Group IIIB cations might precipitate with Group IIIA cations. (D) Signup and view all the answers

What is the expected outcome if the $OH^-$ concentration is not properly controlled during Group III cation analysis?

Both Group III-A and Group III-B, as well as $Mg(OH)_2$, may precipitate together. (A) Signup and view all the answers

Which metal hydroxide is known to form a greyish-green precipitate?

$Cr(OH)_3$ (D) Signup and view all the answers

Which reagent is used to oxidize $Mn(OH)_2$ to $MnO_2$?

$NaOH + Br_2$ (D) Signup and view all the answers

What is the color of the precipitate formed when $H_2S$ reacts with $Zn^{2+}$?

White (B) Signup and view all the answers

Sky blue precipitate formation confirms the presence of which ion?

$Zn^{2+}$ (D) Signup and view all the answers

Which of the following reagents produces a red precipitate with $Co^{2+}$?

Potassium Ferrocyanide + $NH_4Cl$ + $NH_4OH$ (C) Signup and view all the answers

What color is observed in the ether layer during the perchromic acid test for chromium?

Blue (B) Signup and view all the answers

Which reagent is used in the red lead test to detect manganese?

$Pb_3O_4 + H_2O_2$ (C) Signup and view all the answers

What is the appearance of $Al(OH)_3$ precipitate?

White gelatinous (B) Signup and view all the answers

Which test involves the formation of a Prussian blue precipitate?

Ferrocyanide Test for $Fe^{3+}$ (C) Signup and view all the answers

What indicates a positive result for $Fe^{3+}$ using the ammonium thiocyanate ($NH_4SCN$) test?

Blood red solution (D) Signup and view all the answers

Which reagent is used in the lead acetate test for chromium?

Pb Acetate (C) Signup and view all the answers

What is the purpose of adding $HNO_3$ and $H_2O_2$ in the dissolution step for separating ferrous and manganous ions?

To dissolve $Fe(OH)_3$ and $Mn(OH)_2$ (C) Signup and view all the answers

What is the color of the precipitate formed in the Dimethylglyoxime (DMG) test for Nickel ($Ni^{2+}$)?

Red (B) Signup and view all the answers

What is the final product formed when Nickel ($Ni^{2+}$) reacts with KCN, NaOH, $Br_2$ and Water?

$NiO_2$ (B) Signup and view all the answers

What color is observed in the ether layer when the vogel test is positive?

Blue (C) Signup and view all the answers

Flashcards

Role of HNO3

HNO3 oxidizes Fe2+ to Fe3+.

Role of H2S

H2S reduces Fe3+ to Fe2+.