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Questions 2. What is an indicator? What is the purpose of an indicator blank? 3. What is the Mohr method limited to solution of pH values from 6 to 10? Show the chemical reaction involved in solution having pH <6.

Questions 2. What is an indicator? What is the purpose of an indicator blank? 3. What is the Mohr method limited to solution of pH values from 6 to 10? Show the chemical reaction involved in solution having pH <6.

Chemistry
Chemistry
10th Edition
ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Chapter1: Chemical Foundations
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Problem 1RQ: Define and explain the differences between the following terms. a. law and theory b. theory and...
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Subject: Analyyical Chemistry Laboratory ANSWER ONLY QUESTIONS NUMBER 2 AND 3 THANK YOU SO MUCH
Questions 2. What is an indicator? What is the purpose of an indicator blank? 3. What is the Mohr method limited to solution of pH values from 6 to 10? Show the chemical reaction involved in solution having pH <6.
Transcribed Image Text:Questions 2. What is an indicator? What is the purpose of an indicator blank? 3. What is the Mohr method limited to solution of pH values from 6 to 10? Show the chemical reaction involved in solution having pH <6.
Experiment # 7 Determination of Chloride Ion in Drinking Water by Precipitation Titration Method Just as an acid-base system can be used as indicator in acid-base titration, the formation of another precipitate can be used to indicate the completion of a precipitation titration. The Mohr method of precipitation titrations involving silver salts uses chromate ion, CrO., to precipitate brown Ag.CrO,. In the Mohr titration of chloride with silver ion, in which chromate ion is used as the indicator, the first permanent appearance of the reddish silver precipitate is taken as the endpoint of the titration. It is necessary, of course that the precipitation of the indicator occur at or near the equivalence point of the titration. Silver chromate is more soluble (about 8.4 x 10ʻmol/L) than silver chloride (about 1 x 10 mol/L). If silver ions are added to a solution containing a large concentration of chloride ions and a small concentration of chromate ions, silver chloride will first precipitate; silver chromate will not form until the silver ion concentration increases to a large value exceeding the Ksp of silver chromate. If one calculate the concentration of chromate that will lead to precipitation of silver chromate at the equivalence point (Ksp of silver chromate is 2 x 10"), it is 0.02M. Such high concentration can not be used in practice, however, since the yellow chromate ion makes it difficult to observe the formation of colored precipitate. Normally a concentration of 0.005M to 0.01 M is employed. The error caused by using such concentration is quite small. It can be corrected by running an indicator blank or by standardizing the silver nitrate against a pure chloride salt under conditions identical to those used in the analysis. Silver can not be titrated directly with chloride, using a chromate indicator. The silver chromate precipitate, present initially, redissolves only slowly near the equivalence point. However, one can add excess standard chloride solution and then back titrate, using the chromate indicator. Standardization and Titration Reaction: Ag" + Clieo AgCl (aq) Indicator Reaction: 2Ag (a) + CrO,", Ag.CrO. () (aq) Apparatus and Reagents 250-ml Erlenmeyer flask potassium chromate, AR grade phenolphthalein 50.00 ml burette Silver Nitrate solid Sodium chloride, AR grade 50.00 ml pipette 100.00 ml graduated cylinder Analytical balance 600 ml beaker 500 ml amber bottle
Transcribed Image Text:Experiment # 7 Determination of Chloride Ion in Drinking Water by Precipitation Titration Method Just as an acid-base system can be used as indicator in acid-base titration, the formation of another precipitate can be used to indicate the completion of a precipitation titration. The Mohr method of precipitation titrations involving silver salts uses chromate ion, CrO., to precipitate brown Ag.CrO,. In the Mohr titration of chloride with silver ion, in which chromate ion is used as the indicator, the first permanent appearance of the reddish silver precipitate is taken as the endpoint of the titration. It is necessary, of course that the precipitation of the indicator occur at or near the equivalence point of the titration. Silver chromate is more soluble (about 8.4 x 10ʻmol/L) than silver chloride (about 1 x 10 mol/L). If silver ions are added to a solution containing a large concentration of chloride ions and a small concentration of chromate ions, silver chloride will first precipitate; silver chromate will not form until the silver ion concentration increases to a large value exceeding the Ksp of silver chromate. If one calculate the concentration of chromate that will lead to precipitation of silver chromate at the equivalence point (Ksp of silver chromate is 2 x 10"), it is 0.02M. Such high concentration can not be used in practice, however, since the yellow chromate ion makes it difficult to observe the formation of colored precipitate. Normally a concentration of 0.005M to 0.01 M is employed. The error caused by using such concentration is quite small. It can be corrected by running an indicator blank or by standardizing the silver nitrate against a pure chloride salt under conditions identical to those used in the analysis. Silver can not be titrated directly with chloride, using a chromate indicator. The silver chromate precipitate, present initially, redissolves only slowly near the equivalence point. However, one can add excess standard chloride solution and then back titrate, using the chromate indicator. Standardization and Titration Reaction: Ag" + Clieo AgCl (aq) Indicator Reaction: 2Ag (a) + CrO,", Ag.CrO. () (aq) Apparatus and Reagents 250-ml Erlenmeyer flask potassium chromate, AR grade phenolphthalein 50.00 ml burette Silver Nitrate solid Sodium chloride, AR grade 50.00 ml pipette 100.00 ml graduated cylinder Analytical balance 600 ml beaker 500 ml amber bottle
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