Mohrs Salt - Formula, Structure, Preparation, Properties, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:37 PM IST

What is Mohr salt?

Mohr salt, also known as ammonium iron (II) sulfate, is an inorganic mineral compound whose chemical formula is given as (NH₄)₂Fe (SO₄)₂(H₂O)₆. Compounds such as this contain two primary cations, the ammonium cation (denoted by NH₄⁺) and the ferrous cation (denoted by Fe²⁺). Therefore, Mohr's salt can be classified as a double salt consisting of ammonium sulfate and ferrous sulfate. This is an important laboratory reagent because it readily crystallizes and crystals formed by it resist oxidation in the presence of air. Salt full form is Ammonium iron II sulfate.

This Story also Contains

What is Mohr salt?
The properties of Mohr salt and Calculate the molar mass of Mohr salt:
How Mohr salt Is Structured
Mohr's salt preparation
A few salts substitute
1. Sea Salt
2. Himalayan Salt
3. Kala NAMAK (Indian Black Salt)
4. Nutritional Yeast
A wide range of applications for Mohr's Salt

The German scientist Karl Friedrich Mohr is attributed with the name of Mohr salt. Mohr salt dissolves in water to form an aqua complex with the chemical formula [Fe (H₂O)₆]²⁺ (like most other salts of ferrous sulfate). Interestingly, this quo complex has a hexagonal molecular structure. Commonly called mohrite, Mohr salt is a mineral form of sodium chloride.Mohr salt

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The properties of Mohr salt and Calculate the molar mass of Mohr salt:

Mohr salt is a compound that contains Fe (SO₄) (NH₄)₂(SO₄) in chemical formula of Mohr salt. Molecular formula Fe (SO₄) (NH₄)₂(SO₄).6H₂O is given by the hexahydrate form of the metal.

The molar mass of Mohr salt (in its anhydrous state) equals 284.05 grams per mole. Mohr salt has a solubility in water which is demonstrated by its hexahydrate having a molecular mass of Mohr salt 392.13 grams.

Mohr salt exhibits a bluish-green color when subjected to standard conditions for temperature and pressure (often abbreviated as STP). If these conditions are met, it becomes crystalline solid.

In standard conditions, Mohr salt has a density of 1.86 grams per cubic centimeter.

Water dissolves Mohr salt and hexahydrated approximately 269 grams per liter at standard temperature and pressure.

How Mohr salt Is Structured

Mohr salt belongs to the family of double sulfates called Totton salts (or Shonie’s in certain cases). A monoclinic geometry is known to exist in the crystal structure of most members of this family (including Mohr salt). A bonding pattern found in the molecular structure of Mohr salt is characterized by the presence of octahedral centers, composed of iron atoms [Fe (H₂O)₆]²⁺. Furthermore, the ammonium and sulfate ions are known to form hydrogen bonds with these centers.

Mohr's salt preparation

A mixture of ammonium sulfate and hydrated ferrous sulfate (mixed in equal proportions) dissolved in a small amount of sulfuric acid is used to prepare Mohr salt. A crystallization process is then performed on the resulting solution to obtain light green crystals of Mohr's salt. When salt is heated, all the cations and anions present in it are released, which is known as ionization. Some of the common impurities in Mohr's salt include nickel, magnesium, zinc, lead, and manganese. It has been proven that the majority of these impurities are from isomorphous salts.

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A few salts substitute

Humans do not have a problem with moderate amounts of salt. There is a good balance of minerals and water in the human body when sodium levels are low. A vital nutrient for life, it is essential for survival. But, it's better to avoid iodized table salt, which is highly refined and used in most fast foods, stripped of many nutrients. The following 4 alternatives to table salt can be used in place of table salt:

1. Sea Salt

Oceanic salt is commonly referred to as evaporating ocean water. The salt is conciliated in this way using one of the oldest methods. The flavor is stronger than table salt, so you need to use less of it to achieve the same taste in the dishes. The salt has a brighter and cleaner taste than table salt since it does not contain bitter iodine, and it tastes excellent with steaks.

Benefits of Sea Salt: Due to the fact that sea salt delivers less sodium than the common table salt, it delivers fewer health benefits. When harvested from the sea salt fields, sea salt contains a number of minerals that assist in the body's daily functioning.

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2. Himalayan Salt

It comes from Pakistan (the Great Salt Range), home to the oldest salt mine in the world. Himalayan salt is pink in color because it contains the remains of ancient sea life which existed within a dried ocean.

A more subtle taste than sea salt, it has a distinctly crisp and salty taste. Fish, poultry, and vegetables taste great with this beer.

Himalayan salt contains around 84 minerals, including iron, sodium, phosphorus, and magnesium. The iodine in this supplement is easily digestible.

3. Kala NAMAK (Indian Black Salt)

Kala NAMAK looks like rock salt made with South Asian condiments. The magical black color comes from iron sulfide.

This salt's taste is similar to egg yolk in terms of savory sweetness. Combined with chutneys, yogurts, and cooling drinks, it makes a great snack.

There is a benefit to black salt: it contains 3% iron and is used in anemia treatment. Constipation can be reduced by consuming black salt daily.

NCERT Chemistry Notes:

4. Nutritional Yeast

Saccharomyces cerevisiae yeast is used to make this bread. The deactivation of nutritional yeast is accomplished by heating and drying.

Its flavor is similar to Parmesan cheese, with cheesy, nutty, and salty/umami characteristics.

The nutritional yeast is rich in vitamins - B1, B2, B3, zinc, and potassium - and is a good vitamin source. It is usually fortified with folate and vitamin B12 in addition to the nutrients in yeast.

A wide range of applications for Mohr's Salt

In the field of analytical chemistry, Mohr salt is classified as one of the most commonly used compounds as a source of Fe²⁺. Mohr salt is a good source of ferrous ions, as it provides relatively long shelf life and resistance to atmospheric oxidation.

When the pH of the environment is relatively high (the medium is somewhat basic), the oxidation of this compound occurs relatively rapidly.

A very important fact to remember is that, generally, the solutions of Mohr salt are acidic, where the ammonium cations in them can explain this.

Additionally, Mohr salt solutions can be treated with sulfuric acid to prevent ferric iron from reacting with oxygen to form ferrocyanide. The other very important application of Mohr salt in Fricke's dosimeter is that it allows the measurement of high gamma radiation doses.

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Frequently Asked Questions (FAQs)

1. 1. Mohr salt's Structure: What is it?

Mohr salt is known to form crystals with a monoclinic geometrical arrangement. Mohr salt possesses bonding patterns that contain [Fe (H₂O)₆]²⁺ containing octahedral centers. A hydrogen bond forms between these centers and the sulfate and the ammonium ions.

2. 2. Which of the following properties of Mohr salt can you list?

In its crystalline form, Mohr salt is a bluish-green solid at Standard Temperature and Pressure (STP). A standard density of 1.86 g.cm^-3 is found for Mohr salt under standard pressure and temperature conditions. When dissolved in water, Mohr salt (hexahydrate) dissolves nearly 269 grams per liter as a hexahydrate. This is a list of some of the properties of Mohr salt.

3. 3. What are some applications of Mohr salt?

Mohr salt is an ion source widely used in analytical chemistry for securing Fe²⁺ ions. It's beneficial to use this salt as a source of ferrous ions due to its relatively long shelf life and resistance to oxidation under atmospheric conditions. It can also be used as a dosimeter, in which high gamma radiation doses are measured using Fricke's dosimeter.

4. 4. What is the purpose of adding dilute sulfuric acid to Mohr salt aqueous solutions?

The dilute sulphatic acid is normally added to Mohr salt solutions in order to prevent the hydrolysis of iron sulfate.

5. 5. What is the chemical structure of Mohr salt?

One of the properties of Mohr salt is its monoclinic crystal structure. There is evidence that the molecular structure of Mohr salt has octahedral centers that are composed of

[Fe (H₂O)₆]²⁺. Hydrogen bonds are formed between these centers and the ammonium and sulfur ions.

6. Why is Mohr's salt preferred over simple iron(II) sulfate in analytical chemistry?

Mohr's salt is preferred over simple iron(II) sulfate because it's more stable in air and resistant to oxidation. This stability is crucial in analytical procedures where precise concentrations of iron(II) are needed. Additionally, Mohr's salt dissolves easily in water without hydrolysis, ensuring accurate solution concentrations.

7. What is the significance of Mohr's salt in redox titrations?

Mohr's salt is significant in redox titrations because it serves as a reliable primary standard. Its precise composition, stability, and resistance to oxidation make it ideal for standardizing oxidizing agents like potassium permanganate or potassium dichromate. The known concentration of Fe2+ in Mohr's salt solution allows for accurate determination of the concentration of the oxidizing agent.

8. What is the role of Mohr's salt in the Fricke dosimeter?

In the Fricke dosimeter, a radiation detection and measurement device, Mohr's salt plays a crucial role. The dosimeter solution contains ferrous ions (from Mohr's salt), sulfuric acid, and oxygen. When exposed to ionizing radiation, the ferrous ions (Fe2+) are oxidized to ferric ions (Fe3+). The amount of Fe3+ produced is proportional to the absorbed radiation dose, which can be measured spectrophotometrically. Mohr's salt is used because of its stability and the well-defined oxidation of its iron(II) content.

9. How does the presence of ammonium ions in Mohr's salt affect its redox properties?

The ammonium ions in Mohr's salt lower the reduction potential of the Fe2+/Fe3+ couple. This makes the iron(II) in Mohr's salt slightly more difficult to oxidize compared to iron(II) in simple iron(II) sulfate. As a result, Mohr's salt solutions remain stable for longer periods, making them ideal for standardization and titration processes.

10. How does the presence of Mohr's salt affect the pH of an aqueous solution?

When Mohr's salt dissolves in water, it produces a slightly acidic solution. This is primarily due to the hydrolysis of the Fe2+ ions, which can be represented as:

11. How does the structure of Mohr's salt contribute to its stability?

The structure of Mohr's salt contributes to its stability through several factors: 1) The presence of ammonium ions reduces the oxidation potential of iron(II), making it less prone to oxidation. 2) The octahedral coordination of water molecules around the iron(II) ion provides additional stability. 3) The crystal lattice structure formed by the ions creates a stable, ordered arrangement.

12. What role does crystal water play in the structure and properties of Mohr's salt?

The six water molecules (crystal water) in Mohr's salt play crucial roles: 1) They coordinate directly to the iron(II) ion, forming an octahedral complex. 2) They contribute to the overall crystal structure, influencing properties like solubility and melting point. 3) They help stabilize the iron(II) oxidation state by completing its coordination sphere.

13. Why is Mohr's salt considered a coordination compound?

Mohr's salt is considered a coordination compound because it contains a central metal ion (Fe2+) coordinated to ligands. In this case, six water molecules act as ligands, forming an octahedral complex around the iron(II) ion. The resulting [Fe(H2O)6]2+ complex ion is a key component of the Mohr's salt structure, exemplifying its nature as a coordination compound.

14. How does the electronic configuration of iron(II) in Mohr's salt influence its magnetic properties?

The iron(II) ion in Mohr's salt has an electronic configuration of [Ar]3d6. In the octahedral field created by the water ligands, the d-orbitals split, leaving four unpaired electrons. This electronic arrangement makes Mohr's salt paramagnetic, meaning it's attracted to magnetic fields. The paramagnetism of Mohr's salt is stronger than that of many other iron(II) compounds due to its high-spin configuration.

15. How does the concept of crystal field splitting apply to Mohr's salt?

In Mohr's salt, the iron(II) ion is surrounded by six water molecules in an octahedral arrangement. This creates a crystal field that splits the d-orbitals of the iron ion into two energy levels: three lower-energy t2g orbitals and two higher-energy eg orbitals. The energy difference between these levels is called the crystal field splitting energy. In Mohr's salt, this splitting is relatively weak, resulting in a high-spin complex with four unpaired electrons.

16. How does the presence of oxygen affect a solution of Mohr's salt?

Oxygen can gradually oxidize the Fe2+ in Mohr's salt to Fe3+:

17. How does the concept of inner-sphere and outer-sphere coordination apply to Mohr's salt?

In Mohr's salt:

18. How does the concept of hard and soft acids and bases (HSAB) apply to Mohr's salt?

In the context of HSAB theory:

19. Why is Mohr's salt sometimes preferred over ferroin as an indicator in dichromate titrations?

Mohr's salt is sometimes preferred over ferroin as an indicator in dichromate titrations for several reasons:

20. Why is Mohr's salt sometimes used in photography?

Mohr's salt is used in photography, particularly in certain developing processes, due to its reducing properties. The Fe2+ ions in Mohr's salt can reduce silver ions to metallic silver, which forms the image in traditional photographic films and papers. The controlled oxidation of Fe2+ to Fe3+ during this process allows for fine-tuning of the development. Additionally, the stability of Mohr's salt solutions makes them reliable for consistent results in photographic processing.

21. How can you distinguish between Mohr's salt and simple iron(II) sulfate visually?

Mohr's salt forms pale blue-green crystals, while simple iron(II) sulfate (FeSO4·7H2O) forms blue-green crystals. The color difference, though subtle, is due to the presence of ammonium ions in Mohr's salt. Additionally, Mohr's salt crystals tend to be more stable and less prone to oxidation when exposed to air, maintaining their color longer than iron(II) sulfate.

22. How does temperature affect the solubility of Mohr's salt?

The solubility of Mohr's salt increases with temperature. As the temperature rises, more thermal energy is available to overcome the lattice energy of the crystal structure, allowing more ions to enter the solution. However, at higher temperatures, there's also an increased risk of oxidation of Fe2+ to Fe3+, which can affect the salt's stability and usefulness in analytical procedures.

23. What happens to Mohr's salt when it's heated strongly?

When Mohr's salt is heated strongly, several changes occur:

24. Why doesn't Mohr's salt form a deep blue color with hexacyanoferrate(III) ions immediately?

Mohr's salt doesn't form a deep blue color with hexacyanoferrate(III) ions immediately because the reaction requires the oxidation of Fe2+ to Fe3+. In Mohr's salt, the Fe2+ is relatively stable and resistant to rapid oxidation. The reaction eventually occurs, forming the deep blue complex known as Prussian blue, but it's slower compared to other Fe2+ sources. This delayed reaction is actually beneficial in some analytical procedures where a gradual color change is desired.

25. How does the substitution of water molecules in Mohr's salt affect its properties?

Substituting the water molecules in Mohr's salt with other ligands can significantly alter its properties:

26. What is Mohr's salt and why is it called a "double salt"?

Mohr's salt is a complex compound with the formula (NH4)2Fe(SO4)2·6H2O. It's called a double salt because it contains two different cations (ammonium NH4+ and iron Fe2+) and one anion (sulfate SO42-) in a single crystalline structure. This unique composition gives Mohr's salt properties that differ from simple iron(II) sulfate.

27. Why is Mohr's salt sometimes referred to as ferrous ammonium sulfate?

Mohr's salt is often called ferrous ammonium sulfate because this name describes its chemical composition more directly. "Ferrous" refers to iron(II), "ammonium" to the NH4+ ions, and "sulfate" to the SO42- ions. This systematic name highlights the presence of both iron and ammonium in the compound, which is key to understanding its properties and behavior.

28. How does the crystal structure of Mohr's salt compare to that of alums?

Mohr's salt and alums share similar crystal structures, both crystallizing in the monoclinic system. However, there are key differences:

29. What is the significance of Mohr's salt in the determination of Chemical Oxygen Demand (COD)?

Mohr's salt plays a crucial role in the determination of Chemical Oxygen Demand (COD), a measure of water pollution. In this test:

30. What is the relationship between the formula of Mohr's salt and its molar mass?

The formula of Mohr's salt, (NH4)2Fe(SO4)2·6H2O, directly relates to its molar mass:

31. How does the electron configuration of iron in Mohr's salt compare to that in potassium ferricyanide?

The electron configurations differ significantly:

32. What role does entropy play in the dissolution of Mohr's salt in water?

Entropy plays a significant role in the dissolution of Mohr's salt:

33. How does the concept of ligand field theory explain the color of Mohr's salt solutions?

Ligand field theory explains the color of Mohr's salt solutions as follows: