Chemistry Experiment- To Prepare a Pure Sample of Ferrous Ammonium Sulphate
Ferrous Ammonium Sulphate is used in the production of iron blue, as an ingredient in brass colouring baths and iron plating solutions, and as a substitute for ferrous sulphate in a variety of applications.
Iron deficiency can be corrected by incorporating iron sulphate, chelated iron, or ferrous ammonium sulphate into pesticide spray applications or using iron-containing complete analysis fertilisers. The most common and usually least expensive is ferrous ammonium sulphate, which contains 21% iron. The quickest-acting material is ferrous sulphate, which improves turf colour in a matter of days but does not last long.
Table of Contents
Aim of the Experiment
Apparatus Required
Theory
Procedure
Observations
Result
Precautions
Aim of the Experiment
To prepare ferrous ammonium sulphate and submit it for inspection.
Apparatus Required
Conical flask
Tripod stand
Burner
Funnel
Watch glass
Glass rod
Filter paper
Wire gauze
China dish
Ferrous sulphate
Ammonium sulphate
Dilute sulphuric acid
Theory
${{Fe}{S}{O}_{4}{.}{(}{N}{H}_{4}{)}_{2}{S}{O}_{4}{.}{6}{H}_{2}{O}}$ is the formula for ferrous ammonium sulphate. It is made by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in sulfuric acid-containing water. When the solution is crystallised, ferrous ammonium sulphate separates from the solution.
Given below is the chemical reaction
${{Fe}{S}{O}_{4} {+} {(}{N}{H}_{4}{)}_{2}{S}{O}{4} {+} {6}{H}_{2}{O}}$ $\to$ ${{Fe}{S}{O}_{4}{.}{(}{N}{H}_{4}{)}_{2}{S}{O}_{4}{.}{6}{H}_{2}{O} {(Mohr's salt)}}$
In this experiment, the addition of sulfuric acid prevents the hydrolysis of this salt. Ferrous ammonium sulphate is a pale green crystalline compound that, unlike ferrous sulphate, does not effloresce. It is less easily oxidised than FeSO4 and, thus, a better volumetric reagent than ferrous sulphate.
Ferrous Ammonium Sulphate, also known as Mohr's salt, is made by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water with a trace of sulfuric acid. Crystallisation occurs in the solution.
Procedure
Separately weigh 7 g ferrous sulphate and 3.5 g ammonium sulphate.
In a beaker containing dilute sulphuric acid, dissolve ferrous sulphate and ammonium sulphate in water.
Warm the solution gently to obtain a clear solution.
Filter the solution to remove suspended impurities, then concentrate the clear filtrate by heating it in a china dish over a sand bath until crystallisation occurs.
Occasionally stir the solution while it is heating.
Allow the solution to cool slowly and quietly. Crystals of ferrous ammonium sulphate will form in the solution after some time.
Decant the crystals from the mother liquor and wash the crystals with cold water.
Dry the crystals by spreading them on a porous plate or between the folds of filter paper.
Observations
Result
The yield of ferrous ammonium sulphate, known as Mohr's salt, is measured in grams.
Precautions
Allow for slow cooling and avoid disturbing the solution during cooling to obtain high-quality crystals.
The heating of the solution should be done quickly during the process. Because prolonged heating produces ferric ions in addition to ferrous ammonium sulphate.
If the solution is yellow rather than green, the experiment should be repeated.
Lab Manual Questions
1. What are double salts?
Ans. Double salts are ionic compounds that are formed by the combination of two different salt compounds. Complex salts are ionic compounds containing a central metal atom surrounded by ligands linked to it via covalent bonds. Double salts completely dissociate in an aqueous medium.
2. What is the oxidation state of Fe in Mohr’s salt?
Ans. The oxidation state of Fe is +2.
3. Why is dilute sulphuric acid used instead of concentrated sulphuric acid?
Ans. Dilute sulphuric acid is used because the reaction occurs in water and not sulfuric acid thus amount of sulfuric acid should be lower than in water.
4. Why is Mohr’s salt preferred over its counterparts?
Ans. To reduce the oxidation of ferric ions Mohr's salt is preferred.
Viva Questions
1. What type of salt is Mohr’s salt?
Ans. Mohr's salt can be categorised as a double salt of ammonium and ferrous sulfate. It is important to note that Mohr's salt is a common laboratory reagent since it readily undergoes crystallisation, and the crystals formed by it are quite resistant to oxidation in the presence of air.
2. What is the formula of ferrous ammonium sulphate?
Ans. $${{(}{N}{H}_{4}{)}{2}{Fe}{(}{S}{O}_{4}{)}_{2}{(}{H}_{2}{O}{)}{6}}$$
3. What is the colour of ferrous ammonium sulphate?
Ans. The colour of ferrous ammonium sulphate is light green.
4. What kind of test method is ferrous ammonium sulfate?
Ans. In common labs, ferrous ammonium sulfate is used to perform a qualitative chemical analysis to identify a solution's unknown concentration.
5. Why ferrous ammonium sulphate is called Mohr's salt?
Ans. It derives its name from the German chemist Karl Friedrich Mohr, a 19th-century pioneer in the development of titration methodology. Mohr's salt is used in analytical chemistry as the preferred source of ferrous ions due to the solid's extended shelf life and resistance to oxidation.
6. What happens when ammonium ferrous sulphate is heated?
Ans. Ferrous ammonium sulfate forms light green crystals. This salt, when heated, ionises to give all cations and anions present in it.
7. Is ferrous ammonium sulphate soluble in water?
Ans. Ferrous ammonium sulphate is dissolved in water.
8. Why mohr’s salt is a double salt?
Ans. This compound is known to contain two primary cations, namely, the ammonium cation and the ferrous cation. Therefore, Mohr's salt can be categorised as a double salt.
9. Why Mohr's salt is used in titration?
Ans. In this titration, Mohr salt acts as a reducing agent, and potassium permanganate acts as an oxidising agent.
10. What is hydrated ferrous sulphate?
Ans. Iron sulfate heptahydrate is a hydrate that is the heptahydrate form of iron sulfate. It is used as a source of iron in the treatment of iron-deficiency anaemia (generally in liquid-dosage treatments; for solid-dosage treatments, the monohydrate is normally used).
Practical Based Questions
1. Which of the following is a double salt?
(a)Potassium ferrocyanide
(b)Potassium ferricyanide
(c)Ferrous ammonium sulphate
(d)Bleaching powder
Answer: (c)
2. The molar mass of Mohr’s salt is
(a)398 g/mol
(b)276 g/mol
(c)360 g/mol
(d)392 g/mol
Answer: (d)
3.Dilute sulphuric acid is added to the preparation of standard Mohr’s solution
(a)To make the solution acidic
(b)To prevent hydrolysis of Mohr’s salt
(c)To make the solution neutral
(d)To make the solution basic
Answer: (b)
4. The apparatus not used in the preparation of standard Mohr’s salt solution is
(a)Volumetric flask
(b)Burette
(c)Analytical electronic balance
(d)None of these
Answer: (b)
5. Is ammonium sulphate a neutral salt?
(a)True
(b)False
Answer: (b)
6. Is Ferrous ammonium sulphate a primary standard?
(a)True
(b)False
Answer: (a)
7. What is the pH of ammonium sulphate?
(a)7.1
(b)5.6
(c)4
(d)5.5
Answer: (d)
8. Is ferrous ammonium sulfate a reducing agent?
(a)True
(b)False
Answer: (a)
9. Is Ferrous ammonium sulphate harmful?
(a)True
(b)False
Answer: (a)
10. Is Ferrous ammonium sulphate a primary standard?
(a)True
(b)False
Answer: (a)
Conclusion
Ferrous Ammonium Sulphate is known as Mohr’s. It is a light green colour sand-like crystalline solid. It is a hazardous substance, and preparation of ferrous ammonium sulfate is carried out from an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water, containing a little sulphuric acid.
The formula for ferrous ammonium sulfate is $${{Fe}{S}{O}_{4}{.}{(}{N}{H}_{4}{)}_{2}{S}{O}_{4}{.}{6}{H}_{2}{O}}$$. It is made by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in sulfuric acid-containing water. When the solution is crystallised, ferrous ammonium sulphate separates from the solution.
FAQs on To Prepare a Pure Sample of Ferrous Ammonium Sulphate
1. What is Mohr's salt and what is its chemical formula? For the 2025-26 board exams, explain why it is classified as a double salt.
Mohr's salt is the common name for ferrous ammonium sulphate, a crystalline compound used frequently in volumetric analysis. Its chemical formula is FeSO₄.(NH₄)₂SO₄.6H₂O. It is classified as a double salt because it is formed from two different simple salts (ferrous sulphate and ammonium sulphate) and when dissolved in water, it dissociates completely into its constituent simple ions: Fe²⁺, NH₄⁺, and SO₄²⁻.
2. In the preparation of Mohr's salt, why is adding a small amount of dilute sulphuric acid considered an important step?
Adding dilute sulphuric acid is a crucial step to prevent the hydrolysis of ferrous sulphate. Ferrous salts in an aqueous solution have a tendency to hydrolyse, forming basic ferric sulphate or ferric hydroxide, which would appear as an impurity. The acid increases the H⁺ ion concentration, shifting the equilibrium to prevent this hydrolysis and ensuring that only pure ferrous ammonium sulphate crystals are formed.
3. From an exam perspective, what makes Mohr's salt a more suitable primary standard than ferrous sulphate in titrations?
Mohr's salt is preferred over ferrous sulphate as a primary standard for two main reasons:
- Stability: It is highly stable and much less susceptible to oxidation by air from the ferrous (Fe²⁺) to the ferric (Fe³⁺) state compared to ferrous sulphate. This ensures the concentration of the solution remains accurate over time.
- Non-Efflorescent: Unlike hydrated ferrous sulphate (FeSO₄.7H₂O), which can lose water molecules to the atmosphere (efflorescence), Mohr's salt is stable and does not change its composition, allowing for more accurate weighing.
4. During the crystallisation of Mohr's salt, what key precaution related to heating is expected to be followed? Explain the chemical reason for this precaution.
The key precaution is to heat the solution gently and avoid prolonged heating. The chemical reason is that excessive heat can provide the energy needed to oxidise the ferrous ions (Fe²⁺) to ferric ions (Fe³⁺), especially in the presence of atmospheric oxygen. This would lead to the formation of an impure, yellowish sample instead of the desired pale green crystals of pure Mohr's salt.
5. If the solution turns yellow instead of the expected pale green during the experiment, what error does this indicate and how can it be rectified?
A yellow colouration indicates a significant experimental error: the oxidation of ferrous (Fe²⁺) ions to ferric (Fe³⁺) ions. This commonly occurs due to:
- Insufficient addition of dilute sulphuric acid, leading to hydrolysis.
- Overheating or prolonged heating of the solution.
To rectify this, the experiment should be repeated, ensuring that an adequate amount of dilute H₂SO₄ is added at the beginning and that the heating process is gentle and just sufficient to dissolve the salts and concentrate the solution.
6. State the oxidation state of iron in Mohr's salt. Does it change when the salt is dissolved in water?
The oxidation state of iron (Fe) in Mohr's salt is +2. No, the oxidation state does not change when the salt is dissolved in water. As a double salt, it simply dissociates into its constituent ions, which includes the stable Fe²⁺(aq) ion, without undergoing any redox reaction upon dissolution.
7. How does a double salt like Mohr's salt differ from a complex salt like potassium ferrocyanide (K₄[Fe(CN)₆]) in terms of their ionic behaviour in solution?
This is an important conceptual difference for board exams:
- A double salt like Mohr's salt completely loses its identity in an aqueous solution and gives a positive test for all of its constituent ions (Fe²⁺, NH₄⁺, and SO₄²⁻).
- A complex salt like potassium ferrocyanide does not dissociate completely. It yields its counter-ion (K⁺) and a stable complex ion ([Fe(CN)₆]⁴⁻). It will not give a positive test for the central metal ion (Fe²⁺) because it is part of the stable ferrocyanide complex.
8. Write the balanced chemical equation for the crystallisation reaction involved in preparing a pure sample of Mohr's salt.
The balanced chemical equation representing the formation and crystallisation of Mohr's salt from an equimolar solution of hydrated ferrous sulphate and ammonium sulphate is:
FeSO₄ + (NH₄)₂SO₄ + 6H₂O → FeSO₄.(NH₄)₂SO₄.6H₂O (s)
9. List three important properties of Mohr's salt crystals that are relevant for the Class 12 CBSE practical curriculum.
Three important properties of Mohr's salt are:
- Colour and Structure: It is a pale green, crystalline solid.
- Ionic Nature: It is a double salt containing two different cations (Fe²⁺ and NH₄⁺).
- Action in Titrations: It acts as a reliable reducing agent in redox titrations, particularly with oxidising agents like potassium permanganate (KMnO₄).
10. What is a likely 1-mark or 2-mark question that could be asked related to the molar mass of Mohr's salt in the CBSE Class 12 Chemistry exam?
A typical question would be to calculate the molar mass of ferrous ammonium sulphate hexahydrate, FeSO₄.(NH₄)₂SO₄.6H₂O. The calculation is as follows:
Molar Mass = [Fe + S + 4(O)] + [2(N + 4(H)) + S + 4(O)] + [6(2(H) + O)]
= [56 + 32 + 64] + [2(14 + 4) + 32 + 64] + [6(2 + 16)]
= 152 + (36 + 96) + 108
= 152 + 132 + 108 = 392 g/mol.
