Qualitative Analysis of Cations - Group I
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Questions and Answers

Why should cold dilute HCl be used instead of concentrated HCl when working with PbCl2?

  • Cold dilute HCl helps prevent PbCl2 from escaping to subsequent groups. (correct)
  • Concentrated HCl prevents the formation of complexes.
  • Concentrated HCl increases the solubility of PbCl2.
  • Cold dilute HCl is less effective in precipitating PbCl2.

What happens to Pb2+ ions after the addition of dilute HCl?

  • They become insoluble and precipitate immediately.
  • They react with HCl to form a gas.
  • They are reduced to lead metal.
  • They may escape to subsequent groups due to their high Ksp. (correct)

Why is washing the precipitate with cold dilute HCl preferred over using water?

  • It increases the likelihood of peptisation.
  • It renders PbCl2 less soluble by common ion effect. (correct)
  • It helps to dissolve more precipitate.
  • It neutralizes the solution more effectively.

What is the result of heating the group precipitate solution in boiling water?

<p>Only PbCl2 dissolves while others remain precipitated. (A)</p> Signup and view all the answers

What is formed when AgCl is dissolved in dilute NH4OH?

<p>Water-soluble silver ammine complex [Ag(NH3)2]+. (B)</p> Signup and view all the answers

What happens to the black precipitate when dissolved in aquaregia?

<p>It converts to HgCl2. (B)</p> Signup and view all the answers

Why is Sn2+ not available to react with HgCl2 when aquaregia is not properly destroyed?

<p>It is oxidized to Sn4+. (B)</p> Signup and view all the answers

What is the result of adding SnCl2 solution to the first part of the solution after destroying excess aquaregia?

<p>Formation of a white precipitate, Hg2Cl2. (B)</p> Signup and view all the answers

What is the color of the precipitate formed when HgCl2 reacts with KI solution?

<p>Red (C)</p> Signup and view all the answers

Why is HgCl2 represented rather than Hg2+ in the equations?

<p>HgCl2 is weakly ionized. (A)</p> Signup and view all the answers

What happens to AgCl when it is treated with dilute NH4OH?

<p>It dissolves completely into ions. (A)</p> Signup and view all the answers

What is the appearance of the precipitate formed when lead ions are reacted with KI solution?

<p>Yellow precipitate of PbI2. (B)</p> Signup and view all the answers

Why is lead chromate (PbCrO4) insoluble in acetic acid but soluble in dilute mineral acids?

<p>It converts to a soluble form in mineral acids. (B)</p> Signup and view all the answers

What is the result of adding ammoniacal centrifugate to dilute HNO3?

<p>Formation of white precipitate of AgCl. (B)</p> Signup and view all the answers

What color is the precipitate formed from the reaction of Pb2+ ions with K2CrO4?

<p>Yellow. (B)</p> Signup and view all the answers

What additional product is formed when Hg2Cl2 is treated with dilute NH4OH?

<p>Black precipitate of Hg0. (A)</p> Signup and view all the answers

When Pb2+ is reacted with dilute H2SO4, what type of precipitate is formed?

<p>White. (B)</p> Signup and view all the answers

What happens when an additional portion of [Ag(NH3)2]+ is reacted with KI solution?

<p>It results in a yellow precipitate of AgI. (D)</p> Signup and view all the answers

What is the primary reason for adding group reagents systematically in the qualitative analysis of cations?

<p>To precipitate cations of their own group and preceding groups. (D)</p> Signup and view all the answers

Why is HCl preferred over NH4Cl for precipitation in the silver group analysis?

<p>HCl avoids the formation of insoluble bismuth oxychloride. (C)</p> Signup and view all the answers

Which of the following steps should be performed after the precipitation of cations?

<p>Wash the precipitate well with an electrolyte solution. (B)</p> Signup and view all the answers

What is the potential outcome of using water for washing a precipitate?

<p>It encourages peptisation of the precipitate. (C)</p> Signup and view all the answers

What happens if a very large excess of HCl is added during the process?

<p>It can dissolve some of the precipitate back into the solution. (B)</p> Signup and view all the answers

What role does washing the precipitate with an electrolyte solution play?

<p>It assists in the removal of adsorbed impurities. (B)</p> Signup and view all the answers

The sulfidic group reagent NH4Cl + NH4OH + H2S is capable of precipitating which groups?

<p>Cations from groups I, II, III, and IV. (D)</p> Signup and view all the answers

Why is it important to conduct a test for complete precipitation for each group?

<p>To ensure the analytical results are precise and accurate. (B)</p> Signup and view all the answers

Flashcards

Qualitative Analysis of Cations

The process of separating a mixture of ions into groups based on their solubilities.

Group Reagent

A reagent that precipitates a specific group of cations, usually by forming an insoluble compound.

Peptization

A process where a precipitate is broken down into smaller particles, forming a colloidal suspension. This can happen when using pure water to wash a precipitate.

Qualitative Analysis Scheme

A process involving the following steps: precipitation, washing, separation, and confirmation. This systematic approach helps identify ions in a mixture.

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Silver Group

A group of cations that are precipitated by cold, dilute hydrochloric acid (HCl) as chlorides.

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Why a slight excess of HCl is used

The addition of a slight excess of HCl is important to ensure complete precipitation of the silver group cations. It also prevents peptization of the insoluble chlorides.

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Why a large excess of HCl is avoided

Adding a large excess of HCl should be avoided as it can dissolve the precipitate. This could result in incomplete separation and inaccurate results.

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Why is cold dilute HCl used to precipitate Group I cations?

Lead chloride (PbCl2) is more soluble in hot water than in cold water. To prevent the loss of PbCl2 to subsequent groups, it is crucial to use cold dilute HCl during precipitation.

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Why is the solution vigorously shaken after adding cold dilute HCl?

PbCl2 can form a supersaturated solution, leading to its escape to subsequent groups. Vigorous shaking ensures complete precipitation and prevents the formation of a supersaturated solution.

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Why can Pb2+ escape from Group I to Group II?

PbCl2 has a high Ksp, meaning it readily dissolves. Shaking and cold dilute HCl minimize PbCl2 loss, but some may still escape to Group II.

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Why is the precipitate washed with cold dilute HCl?

Washing the precipitate with cold dilute HCl reduces the solubility of PbCl2 by the common ion effect. It also prevents peptization, where the precipitate redissolves.

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How are Group I cations separated?

Boiling water dissolves PbCl2, but not AgCl or Hg2Cl2. This allows for the separation of these cations.

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Lead Chloride Precipitation

Lead chloride (PbCl2) forms needle-like crystals when a hot solution containing lead ions is cooled.

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Lead Chromate Formation

Lead chromate (PbCrO4) is a bright yellow precipitate that forms when a solution containing lead ions reacts with potassium chromate (K2CrO4).

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Lead Chromate Solubility

Lead chromate (PbCrO4) is insoluble in acetic acid but dissolves in dilute mineral acids like nitric acid (HNO3).

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Lead Iodide Precipitation

Lead iodide (PbI2) is a yellow precipitate that forms when a solution containing lead ions reacts with potassium iodide (KI).

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Lead Sulfate Formation

Lead sulfate (PbSO4) is a white precipitate that forms when a solution containing lead ions reacts with dilute sulfuric acid (H2SO4).

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Silver Chloride Reprecipitation

Silver chloride (AgCl) reprecipitates as a white solid when dilute nitric acid (HNO3) is added to a solution containing the diamminesilver(I) ion ([Ag(NH3)2]+).

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Silver Iodide Precipitation

A yellow precipitate of silver iodide (AgI) forms when potassium iodide (KI) is added to a solution containing the diamminesilver(I) ion ([Ag(NH3)2]+).

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Silver Iodide Precipitation Priority

The addition of potassium iodide (KI) to a solution containing the diamminesilver(I) ion ([Ag(NH3)2]+) results in the formation of a yellow precipitate of silver iodide (AgI), even though the solution already contains chloride ions.

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What happens to the black and white precipitates in aqua regia?

The black precipitate of HgO and the white precipitate of HgNH2Cl are both converted to HgCl2, a water-soluble compound.

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What happens when SnCl2 is added to HgCl2?

Adding excess SnCl2 to a solution of HgCl2 will first form a white precipitate of Hg2Cl2, which turns to grey and then black due to the formation of HgO.

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Why is HgCl2 written as HgCl2 in equations and not Hg2+?

HgCl2 is weakly ionized in solution, meaning it doesn't readily dissociate into Hg2+ and Cl- ions.

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Why should HgCl2 not be evaporated to dryness?

Evaporation of the solution to dryness while destroying aqua regia can lead to the complete loss of mercury because HgCl2 is volatile.

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Why is it important to destroy the excess aqua regia before proceeding with the next step?

Aquaregia can oxidize Sn2+ to Sn4+, preventing the reaction of Sn2+ with HgCl2. Make sure it's destroyed properly!

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Study Notes

Qualitative Analysis of Cations

  • Cations are categorized into groups based on the difference in solubility of chlorides, sulphides, hydroxides, and carbonates.
  • A systematic approach is used, adding group reagents to precipitate specific cation groups.
  • A test for complete precipitation is crucial before separating subsequent groups.
  • Washing precipitates with an electrolyte solution like HCl is needed to remove adsorbed impurities and prevent peptization, a transformation of the precipitate into a colloidal state.

Group I (Silver Group) Cations

  • This group includes Lead (Pb2+), Silver (Ag+), and Mercurous (Hg22+) ions.
  • The group reagent is cold dilute HCl
  • The precipitate forms chlorides (AgCl, PbCl2, Hg2Cl2) which are white in color.
  • An excess of HCl should not be used to avoid increased solubility of chlorides.
  • A cold dilute HCl solution is used and not heated to avoid escaping the PbCl2 ions.
  • The solution must be thoroughly shaken for a few minutes post addition of the reagent to ensure complete precipitation and prevent the formation of supersaturated solutions and escape of PbCl2 to subsequent groups.

Steps of Analysis

  • Precipitation: Adding a specific reagent to precipitate the cations according to group separation.
  • Washing: Wash the precipitate with dilute cold HCl to remove adsorbed impurities and prevent peptization avoiding water washing.
  • Separation: The precipitate is heated or dissolved in another solution to obtain individual ions
  • Confirmation: Separate and identify the ions by specific analyses, such as reacting a portion of the solution with other reagents to obtain confirmatory precipitates for the particular cation being identified

Washing

  • Washing precipitates with dilute cold HCl is preferred over water due to the common ion effect, making PbCl2 less soluble, and preventing peptization.

Separation (Group I)

  • The mixture of precipitates (AgCl, PbCl2, Hg2Cl2) is heated with water, with only PbCl2 dissolving.
  • The precipitate is then dissolved with dilute NH4OH, converting AgCl into a soluble silver ammine complex ([Ag(NH3)2]+), while Hg2Cl2 forms an insoluble black mixture of aminomercuric chloride and finely divided mercury

Confirmation (Lead)

  • Lead is confirmed using potassium dichromate (K2CrO4) and acetic acid forming a yellow precipitate (PbCrO4) which is insoluble in acetic acid but dissolves in diluted nitric acid (HNO3).
  • When heated, the yellow precipitate is soluble in water
  • When hot solution is reacted with dilute H2SO4 white precipitate PbSO4 is formed

Confirmation (Silver)

  • The ammoniacal centrifugate (containing Ag+ as [Ag(NH3)2]+) is treated with nitric acid (HNO3 to form a white precipitate AgCl).
  • The ammoniacal centrifugate is added with  KI solution to produce a yellow AgI precipitate.

Confirmation (Mercurous)

  • The black precipitate is dissolved in aquaregia (3HCl + 1HNO3).
  • This black precipitate is converted to HgCl2 and water soluble,
  • The solution can be divided into two portions, then adding SnCl2 solution yield a black precipitate of Hg°.
  • Excess aquaregia can be destroyed by boiling, if the solution is heated for too long, the HgCl2 can be lost as it is volatile.

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Pharmaceutical Analytical Chemistry-I Qualitative Analysis of Cations PDF

Description

This quiz focuses on the qualitative analysis of Group I cations, which include Lead, Silver, and Mercurous ions. It covers the methods used for precipitation, the importance of using cold dilute HCl, and the characteristics of the resulting chlorides. Test your understanding of the systematic approach required in this analysis.

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