Determination of Strength of KMn O4 By Mohr\'s salt-Copy PDF

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Determination of Strength of KMnO₄ solution The strength of KMnO4 solution can measured by • •

Oxalic acid Ferrous ammonium sulphate (Mohr’s salt)

Some Important Terms in Titration 1. Standard solution A solution whose concentration is known, is called a standard solution. The substance used to prepare a standard solution is called the primary standard. Oxalic acid and sodium carbonate are some examples. 2. Concentration of a solution Concentration of a solution is defined as the amount of a solute present in a definite volume of the solvent. Concentration of a solution can be expressed in different ways. •

Normality: Normality of a solution is defined as the number of gram equivalent of solute per litre of the solution. It is denoted by ‘N’.

•

Molarity: Molarity of a solution is defined as the number of gram moles of the solute per litre of the solution. It is denoted by ‘M’.

3. End point of a titration The end point of a titration is the point at which the reaction between the titrant and the analyte becomes complete. Generally the endpoint of a titration is determined using indicators. In some cases, either the reactant or the product can serve as the indicator. A best example is the redox titration using potassium permanganate.

Titrations can be classified as: • • • •

Acid-Base Titrations or Acidimetry and Alkalimetry Oxidation-Reduction Titrations or Redox Titrations Precipitation Titrations Complexometric Titrations Oxidation-Reduction Titrations or Redox Titrations

In redox systems, the titration method can be adopted to determine the strength of a reductant/oxidant using a redox sensitive indicator. Redox titrations involving potassium permanganate are called permanganometric titrations. In these reactions, MnO4- ions acts as the self indicator. Titration of Potassium permanganate (KMnO4) against Mohr’s salt solution. A. Theory View Stage 1: Preparation of standard solution of Mohr's salt: • We will prepare 250 ml from Mohr’s salt with concentration 0.05 M. • The molecular mass of Mohr's salt is, FeSO4.(NH4)2SO4.6H2O= 392 • Weight of Mohr's salt required to prepare 1000 ml of 1 M solution = 392 g. • weight of Mohr's salt required to prepare 250 ml 0.05 M Mohr's salt solution

Stage 2: Determination of strength of KMnO4 ✓ In this titration, potassium permanganate is the oxidizing agent and Mohr’s salt is the reducing agent. ✓ Mohr’s salt is a double salt of ferrous sulphate and ammonium sulphate and its composition is FeSO4.(NH4)2SO4.6H2O. It is a primary standard.

✓ Ferrous ions of Mohr’s salt undergo hydrolysis in aqueous solution. To prevent the hydrolysis, Conc. H2SO4 needs to be added to the Mohr’s salt crystals during the preparation of its standard solution. ✓ In this titration, the MnO4- ion is reduced to Mn2+ in the presence of acid and Fe2+ ions of Mohr’s salt is oxidized to Fe3+ ✓ The chemical reaction that occurs in this titration can be represented by the following chemical equations. Molecular equation

Ionic equation

Balanced chemical equation From the overall balanced chemical equation, it is clear that 2 moles of potassium permanganate react with 10 moles of Mohr’s salt.

B. Practical View 1. Tools, chemicals and equipment Chemicals

Tools and Glasses

Equipment

Burette 25ml or 50ml Burette Stand Conical Flask 250 ml Glass Rod KMnO4 Solution Watch Glass Mohr’s crystals Volumetric flask 250 ml Con.c H2SO4

Digital Balance Beaker 250 ml

Diluted H2SO4 Funnel Distilled water Pipette 10 ml Spatula Dropper Cylinder 50 ml 2. Preparation of standard solution of Mohr’s salt 2.1. Measure 4.9 gram from Mohr’s salt (see theory part) on the digital balance. 2.2. Add to 5 ml from Con.c H2SO4 then 15 ml distilled water to last weight of crystals and dissolving it well 2.3. Complete the volume to 250 ml with distilled water in volumetric flask. 2.4. Shaking gently the volumetric flask to make sure the distribution of Mohr’s crystals.

3. Titration KMnO4 against Mohr’s salt as the following figure KMnO4

10 ml 0.05M of Mohr's salt 10 ml diluted H2SO4

Color will change from colorless to Pink color

4. Record your observation at which the color change from colorless to pink color (KMnO4 color) in the following table Trial

Vi

Vf

V

1 2 3 volume 5. Calculate the concentration of KMnO4 as following: volume of KMnO4 * Molarity of KMnO4 volume of Mohr * Molarity of Mohr

=

moles of KMnO4 moles of Mohr

=

2 10

Molarity of KMnO4 = ........ M

Strength of KMnO4 = Molarity of KMnO4 * Molar mass of KMnO4 Strength of KMnO4 = Molarity of KMnO4 * 158 =

g/l

Thank Thankss Dr. Alaa H Hassanie assanie assanienn...