24 Questions
Why is it necessary to make the original solution almost neutral before adding a buffer solution?
To ensure the buffering action is achieved
What happens if the metal ion concentration is too high in a titration?
The end point is very difficult to discern
What is a fault to be guarded against when adding an indicator in a titration?
Adding too much indicator
What is the typical amount of indicator mixture used in a titration?
30-50 mg of a 1% solid mixture
What is the consequence of adding too much indicator in a titration?
The end point anticipation is lost
Why is it important to carefully add alkali to an acid solution containing a metallic ion?
To prevent the metal hydroxide from precipitating
What is the typical volume of solution used in a titration?
50-150 mL
What is the purpose of using a buffering medium in a titration?
To achieve the requisite buffering action
What is the equivalent weight of the ferrous compound FeSO4.(NH4)2SO4.6H2O?
392
What is the oxidation number of Fe2+ in the reaction Fe2+ → Fe3+?
2
What is the purpose of calculating the equivalent weight of a substance?
To determine the normality of a solution
What is the normality of the ferrous solution containing 1,176g of FeSO4.(NH4)2SO4.6H2O in 30.00 ml?
0.1000 meq/ml
What is the equivalent weight of the dichromate K2Cr2O7?
K2Cr2O7/6
What is the purpose of calculating the normality of a solution?
To determine the reacting quantities in titrimetric analysis
What is the oxidation number of Cr2O72- in the reaction Cr2O72- → Cr3+?
6
What is the volume of the dichromate solution equivalent to 1.000 ml of the ferrous solution?
5.00 ml
Why is it important to perform Volhard's method in an acidic medium?
To prevent the precipitation of iron ions as hydrated oxide
What is the purpose of adding a common ion to the wash liquid in precipitation reactions?
To diminish solubility losses
What is the effect of pH on the solubility of the salt of a weak acid?
Solubility increases with increasing pH
What is the principle behind the formation of a colored complex in Volhard's method?
Formation of a metal-ligand complex
What is the role of the indicator in Volhard's method?
To form a colored complex with the ferric ammonium sulfate
What is the effect of the diverse-ion effect on the solubility of a precipitate?
Solubility increases in the presence of salts containing no ions in common with the precipitate
What is the reaction that occurs at the end point of Volhard's method?
Fe3+ + CNS- → Fe(CNS)3
What is the purpose of using potassium thiocyanate solution in Volhard's method?
To react with the excess of silver nitrate
Study Notes
Titration and Precipitation Reactions
- In acid-base titrations, the original solution should be made almost neutral by adding sodium hydroxide, ammonium hydroxide, or dilute acid before adding the buffer solution.
- When neutralizing an acid solution containing a metallic ion with alkali, care must be taken to avoid precipitating the metal hydroxide.
Concentration of Metal Ion and Indicator
- Most titrations are successful with 0.25 millimole of the metal ion in 50-150 mL of solution.
- If the metal ion concentration is too high, the end point may be difficult to discern, and it may be necessary to start with a smaller portion of the test solution and dilute it before adding the buffering medium and indicator.
- Too much indicator should be avoided, as it can intensify the color during titration and exhibit dichroism, making it difficult to discern the end point.
Attainment of End Point and Solubility
- To ensure complete precipitation, a common ion can be used in the wash liquid to diminish solubility.
- The diverse-ion effect causes precipitates to show increased solubility when salts containing no ions in common with the precipitate are present.
- Solubility of the salt of a weak acid depends on the pH of the solution.
Hydrolysis, Complexation, and Precipitation Titration
- Hydrolysis and complexation can affect solubility.
- Precipitation titration involves reacting a substance with a standard solution of a precipitating agent, and the end point is marked by a color change with an indicator.
Methods of Precipitation Titration
- Volhard's method is used for determining anions like halides, phosphate, and chromate in acidic medium using silver ions.
- In Volhard's method, the leftover silver nitrate reacts with potassium thiocyanate solution, and the excess thiocyanate reacts with an indicator to form a complex.
Equivalents, Normalities, and Oxidation Numbers
- Equivalent weights and normalities are traditional concepts used in titrimetric analysis, especially in reduction-oxidation titrations.
- Oxidation numbers are required to understand the equivalent weights of substances involved in redox reactions.
- Examples of equivalent weights are provided for various substances, including FeSO₄.7H₂O, SnCl₂, H₂C₂O₄.2H₂O, KHC₂O₄.H₂O, H₂S, Na₂S₂O₃.5H₂O, and H₂O₂.
Learn about acid-base titrations and precipitation reactions in chemistry, including tips for neutralizing acid solutions and avoiding metal hydroxide precipitation.
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