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Preparation of Mohr’s Salt - Introduction, Preparation, Precautions, Applications, Practice Problems & FAQs

Have you done grocery shopping? 

In the mall, you must have seen many flavouring substances available, they basically add a taste to a particular thing. Also, some of the substances may occasionally emit an unpleasant odour that may be detrimental to the taste. Sometimes these additive substances may create health problems as well. 

In dairy products for example cheese, fragrances or flavours are frequently added to enhance their flavour and appeal. But occasionally, these compounds might emit a disagreeable smell that may ruin the flavour of the cheese. In order to get rid of this smell and keep the cheese's unique flavour, ferrous ammonium sulphate is utilised.

So let us study how this important compound is prepared in the laboratory and what materials are required for the preparation of hydrated ferrous ammonium sulphate commonly known as Mohr’s salt.


TABLE OF CONTENT

  • Introduction to Mohr’s Salt 
  • Preparation of Mohr’s Salt
  • Applications of Mohr’s Salt
  • Practice Problems
  • Frequently asked questions-FAQ

Introduction to Mohr’s Salt 

Ferrous Ammonium Sulphate, a crystalline solid with a pale green tint resembling sand, is also known as Mohr's salt (). The production of ferrous ammonium sulphate involves mixing hydrous ferrous sulphate and ammonium sulphate in water at an equimolar ratio while adding a little amount of sulfuric acid.

The name of Mohr’s salt was inspired by a German scientist named Karl Friedrich Mohr. Mohr's salt easily dissolves in water to form an aqueous complex solution which has the chemical formula. The geometry of this complex is octahedral. In the mineral form, the name of Mohr’s salt is Mohrite.

Mohr’s salt is also a double salt which is a mixture of more than one simple salt. If dissolved in an aqueous solution, it dissociated completely. It dissociates into , , ions and gives their individual chemical tests.

Preparation of Mohr’s Salt 

The ferrous ammonium sulphate (Mohr’s salt, ) is prepared using ammonium sulfate and ferrous sulfate. It is made by dissolving hydrated ferrous sulphate and ammonium sulphate in an equimolar ratio in water that contains a small amount of sulfuric acid. Ferrous ammonium sulphate separates from the solution as a result of the crystallisation of the solution.

The sulfuric acid is added to stop this salt from hydrolysing. In contrast to ferrous sulphate, ferrous ammonium sulphate is a pale green crystalline substance. It is a superior volumetric reagent to ferrous sulphate because it is less easily oxidised than .

The colour of Mohr’s salt is light green, it is transparent and has an octahedral shape of crystals. It dissolves in water to produce an acidic solution that colours blue litmus paper red and causes effervescence when combined with sodium bicarbonate, which releases carbon dioxide.

Laboratory preparation of Mohr’s salt: 

The materials which are required are as follows:

  1. Conical flask
  2. Glass rod
  3. Burner
  4. Tripod stand
  5. Watch glass
  6. Funnel
  7. Wire gauze
  8. Filter paper
  9. Beaker
  10. Ammonium sulfate
  11. Ferrous sulfate
  12. Dilute sulphuric acid

The procedure is as follows: 

  • To prevent contamination, first wash all glassware with distilled water.
  • Separately weigh 3.5 g of ammonium sulphate and 7 g of ferrous sulphate.
  • of water should be boiled for minutes in another beaker.
  • After the water has boiled, slowly pour it into the beaker containing the ammonium and ferrous sulphates. Salts must be continuously stirred until they are fully dissolved.
  • Filter the resulting solution. When you have the filtrate, heat it in a beaker until it reaches the point of crystallisation.
  • Allow the beaker to cool for roughly hours.
  • The crystallisation of mohr's salt occurs. Utilize filter paper to gather these crystals.
  • Let the crystals dry thoroughly. On a balance, weigh them, and record the result.


The light green crystals of Mohr’s salt are observed which are soluble in water and can turn blue litmus paper to red showing the acidic nature of Mohr’s salt and having an octahedral crystal shape. 

Precautions:

  • To obtain crystals of high quality, allow for a gradual cooling process without disturbing the solution.
  • Heating the fluid should only be done briefly during the process. due to the formation of ferric ions and ferrous ammonium sulphate after prolonged heating.
  • Imagine that the experiment needs to be run again if the solution is yellow rather than green.

Applications of Mohr’s Salt

  • In the metal industries, Mohr’s salt is used to make iron blue, as a component of baths having brass colour and iron plating solutions which is a substitute for ferrous sulphate in a variety of applications.
     
  • Mohr’s salt can be added to pesticide spray applications to correct an iron deficit, as can complete analysis of fertilisers that also contain iron.
  • Mohr's Salt is a compound which acts as a source of ions. Mohr's salt has a relatively longer shelf life and is resistant to oxidation from atmospheric exposure which are the major advantages of Mohr’s salt being the source of ferrous ions.
     
  • The oxidation of this chemical is well-known to take place rather quickly when the pH of the environment is high (the medium is somewhat basic). Most importantly, it should be noted that Mohr's salt solutions typically have a slightly acidic pH, which can be attributed to the ammonium cations that are present in them. 
     
  • Additionally, it should be mentioned that sulfuric acid can be added to Mohr's salt solutions to stop the ferrous ions from oxidising into the ferric ions. 
     
  • Mohr's salt is used in Fricke's dosimeter. This device is used to measure the high doses of gamma radiation.

Practice Problems:


1. The molar mass of Mohr's salt comes out to be_____________

A.
B.
C.
D.

Answer: (B)
We have studied the molar mass of few important elements from which we can easily calculate the molar mass of Mohr’s salt.
Molar mass of
Molar mass of
Molar mass of
Molar mass of  
Using the above information, we can calculate the molar mass of Mohr’s salt () which comes out to be 392 .
Molar mass of

2. Standard Mohr's salt solution is prepared by mixing dilute sulphuric acid with the salt solution ______.

A. to neutralise the solution
B. to avoid Mohr's salt hydrolysis
C. to make the solution acidic
D. to make the solution basic

Answer: (B)
The addition of dilute sulfuric acid stops the hydrolysis of ferrous sulphate. This makes the overall solution acidic. Avoid overheating the water when dissolving the salt mixture in it. This is to stop the conversion of light green ions to ions (yellow).

3. The oxidation state of iron in Mohr’s salt is calculated as_______.

A. +2
B. +3
C.  0
D. +4

Answer: (A)
The given compound is Mohr’s salt i.e., .

Let us suppose the oxidation state of Fe be x.
The oxidation number of is +1
The oxidation number of is −2
The oxidation number of H is +1
The oxidation number of O is −2

The oxidation number of Fe in is calculated as,


Hence, the oxidation state of iron comes out to be +2 in .

4. The equipment that is not used to prepare the Mohr's salt solution in the laboratory is______________.

A. conical flask
B. analytical electronic balance
C. burette
D. volumetric flask

Answer: (C)
The equipment that is not used to prepare the typical Mohr's salt solution in the laboratory is the burette. Conical flask, analytical electronic balance, and volumetric flasks all are used during the preparation of Mohr’s salt. .

Frequently Asked Questions:


1. What uses do you have for Mohr's salt?

Answer: Mohr's salt is a well-known source of ions in the realm of analytical chemistry. Because it has a reasonably long shelf life and is resistant to oxidation brought on by contact with the atmosphere, Mohr's salt is a good source of ferrous ions. Fricke's dosemeter, which aids in the measurement of large doses of gamma radiation, also uses Mohr's salt.

2. Why does titration use Mohr's salt with potassium permanganate?

Answer: Ferrous ammonium sulphate is the chemical name for Mohr's salt. It should be emphasised that in this titration, potassium permanganate acts as an oxidising agent and Mohr salt as a reducing agent.

3. What distinguishes complex salt from double salt?

Answer:

Double salt Complex salt
These only have a solid state existence and split up into separate species in their solution. Exist both in the solid and solution states, they maintain their identity.
When they are dissolved, they lose their individuality. They retain their individuality when disintegrated.
 In double salt, the metal atom/ion exhibits its normal valency. The amount of negative ions or molecules surrounding the main metal atom in coordination compounds is more than its normal valency.
Example: Example:

4. Why is diluted sulfuric acid added to Mohr's salt aqueous solutions?

Answer: To avoid the ions in the Mohr salt solution from being oxidised (to ), dilute must be added while making the standard solution of Mohr salt, which is composed of. Also, to stop the hydrolysis of ferrous sulphate, diluted sulphuric acid is often added to aqueous solutions of Mohr's salt.

Related Topics

Potassium permanganate Potassium dichromate
mohr's salt titration with KMnO4 Water
Hydrogen Important compounds of Copper

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