Mohr’s Salt

Mohr’s Salt is named after the German chemist Karl Friedrich Mohr, who synthesized it in 1855. In analytical chemistry, Mohr’s salt is frequently used as a primary standard to standardize solutions of other substances, especially when titrating permanganate solutions. In addition, it finds use as a fertilizer in agriculture and various laboratory applications. In this article, we will discuss the various concepts related to Mohr’s Salt.

What is Mohr’s Salt?

Mohr’s salt, also known as Ferrous Ammonium Sulfate, is a chemical compound with the formula (NH₄)₂Fe(SO₄)₂·6H₂O. It is a double salt of iron(II) sulfate and ammonium sulfate. The chemical structure shows that it contains two ammonium ions (NH₄⁺), one iron(II) ion (Fe²⁺), two sulfate ions (SO₄²⁻), and six water molecules (H₂O) in its hydrated form.

Note: Mohr’s salt is commonly utilized as a laboratory reagent due to its easy crystallization and the production of crystals that exhibit high resistance to oxidation when exposed to air.

Structure of Mohr’s Salt – IIT JEE

The Chemical Formula of Mohr’s Salt is (NH₄)₂Fe(SO₄)₂·6H₂O. On breaking this structure we get,

• Ammonium Ions (NH₄⁺ )
• Iron (II) Ions (Fe⁺⁺)
• Sulfate Ions (SO₄^—)
• Water Molecules (H₂O)

These components come together with ions to form the complex structure of Mohr’s salt with a monoclinic geometry, that looks like the given below figure.

Formula for Mohr’s Salt

The Chemical Formula of Mohr’s Salt is (NH₄)₂Fe(SO₄)₂·6H₂O.

Preparation of Mohr’s Salt

Mohr’s salt ((NH₄)₂Fe(SO₄)₂·6H₂O), can be prepared through a reaction between ferrous sulfate (FeSO₄) and ammonium sulfate ((NH₄)₂SO₄) in the presence of water. Here’s a simple method for the preparation:

Materials Required

• Ferrous sulfate (FeSO₄·7H₂O)
• Ammonium sulfate ((NH₄)₂SO₄)
• Distilled water
• Glassware (flasks, beakers, stirring rod)
• Heat source

Procedure of Preparation

The steps to prepare Mohr’s Salt is mentioned below:

Step 1: Calculate the necessary quantities of ammonium and ferrous sulfates. These two compounds have a molar ratio of 1:2.

Step 2: Dissolve the measured amount of ferrous sulfate in a suitable volume of distilled water. Stir the solution until the compound is completely dissolved.

Step 3: Similarly, dissolve the measured amount of ammonium sulfate in another container with distilled water. Stir until the ammonium sulfate is fully dissolved.

Step 4: Now, combine the two solutions into a single container, and stir thoroughly. This mixing will lead to the formation of Mohr’s salt.

Step 5: Allow the solution to cool. During cooling, Mohr’s salt will begin to crystallize. The crystals can be separated from the solution through filtration.

Step 6: Wash the crystals with a small amount of cold water to remove any impurities.

Step 7: Dry the crystals by air-drying. Once dried, Mohr’s salt can be stored in a dry place.

Chemical Reaction

The chemical reaction involved in the preparation of Mohr’s Salt is as follows:

FeSO₄ + (NH₄)₂SO₄ + 6H₂O → Fe(NH₄)₂(SO₄)₂.6H₂O

Properties of Mohr’s Salt

Ferrous ammonium sulfate hexahydrate, also known as Mohr’s salt ((NH₄)₂Fe(SO₄)₂·6H₂O), has several interesting physical and chemical characteristics. The properties of Mohr’s Salt can be understood easily under the following heads below:

Physical Properties

The physical properties of Mohr’s Salt can be understood by the table below:

Properties of Mohr’s Salt
Chemical Formula	(NH4)2Fe(SO4)2·6H2O
Chemical Name	Ferrous ammonium sulfate hexahydrate
Physical State	Crystalline solid
Crystal Structure	Monoclinic
Color	Light Green
Solubility in Water	Soluble (increases with temperature)
Magnetic Properties	Exhibits magnetic properties due to iron(II) ions
Color Change upon Oxidation	May change from green to brown upon exposure to air

Chemical Properties of Mohr’s Salt

The chemical properties of Mohr’s Salt are as follows:

1. Reaction with Alkalis: Mohr’s salt reacts with alkalis to form insoluble hydroxides. For example:

(NH₄)₂Fe(SO₄)_2.6H₂O + 2NaOH → Fe(OH)₂ + 2(NH₄​)₂SO₄ + 6H₂O

2. Formation of Ferric Salt: Mohr’s salt can be oxidized to form ferric sulfate in the presence of oxidizing agents.

(NH₄)₂Fe(SO₄)_2.6H₂O + Br₂ → Fe₂(SO₄)₃ + 2NH4Br + 6H2O

3. Decomposition upon Heating: Mohr’s salt undergoes thermal decomposition upon heating, releasing ammonia, sulfur dioxide, and water vapor.

(NH₄)₂Fe(SO₄)₂.6H₂O→FeSO₄ + (NH₄)₄SO₄ + 6H₂O

4. Oxidation by Air: In the presence of air, the iron(II) ions in Mohr’s salt can oxidize to iron(III) ions (Fe³⁺). This oxidation is often observable as a color change from green to brown.

Double Salt

A double salt is a salt that contains two or more different cations or anions.

• Double salts exist in solid state but dissociate into constituent ions on dissolution in water.
• The analysis of double salts can be easily done by determining the ions that are present in the aqueous solution.
• Examples include Potash Alum (K₂SO₄. Al₂(SO₄)₃.24H₂O), Mohr’s Salt((NH₄)₂Fe(SO₄)₂.6H₂O)

Learn, Family of Salts

Uses of Mohr’s Salt

The uses of Mohr’s Salt are mentioned below:

• Mohr’s salt is used as a reducing agent in various redox reactions.
• Mohr’s salt finds applications in laboratories as a versatile reagent.
• The compound is used as a fertilizer in agriculture.
• In some chemical reactions, Mohr’s salt can act as a catalyst .
• Mohr’s salt reacts with alkalis to form insoluble hydroxides.

Mohr’s Salt: Frequently Asked Questions

What is Mohr’s Salt?

Mohr’s salt, also known as Ferrous Ammonium Sulfate, is a chemical compound with the formula (NH₄)₂Fe(SO₄)₂·6H₂O.

What is Mohr’s salt used for?

Mohr’s salt is used in analytical chemistry for standardization, as a reducing agent in redox reactions, in laboratory experiments, and as a fertilizer in agriculture.

Is Mohr’s salt a Double Salt?

Yes, Mohr’s Salt is a double salt.

What happens when Mohr’s Salt is heated?

Mohr’s salt undergoes thermal decomposition upon heating, releasing ammonia, sulfur dioxide, and water vapor.

What is the Crystal Structure of Mohr’s salt?

Mohr’s salt has a monoclinic crystal structure.

What is the color of Mohr’s salt?

The color of Mohr’s Salt is Bluish-Green.

What is the Chemical Name for Mohr’s salt?

Ammonium iron(II) sulfate is the chemical name for Mohr’s Salt.

What is the Molar Mass of Mohr’s salt?

The molar mass of Mohr’s salt (in its anhydrous state) is equal to 284.05 grams per mole. While the hexahydrate of Mohr’s salt is known to have a molar mass of 392.13 grams per mole.