How to Prepare Mohrs Salt Balanced Chemical Equation Principle and Procedure
The preparation of Mohr’s salt, also known as ammonium iron(II) sulphate hexahydrate, is a vital laboratory process for students in chemistry, especially in Class 12 practicals. Understanding how to prepare this double salt not only helps in mastering titrations, but also introduces concepts like crystallization and salt stability. This article details the materials, procedure, precautions, and important facts related to the preparation of Mohr’s salt, supporting learning as per NCERT and other academic standards.
What is Mohr’s Salt?
Mohr’s salt is a well-known inorganic compound, often used as a reliable primary standard in chemical analysis. It has the formula \( (NH_4)_2Fe(SO_4)_2 \cdot 6H_2O \) and forms pale green crystals. This double salt is composed of ferrous sulphate and ammonium sulphate in equimolar amounts.
Why Is Mohr’s Salt Called a Double Salt?
- A double salt is formed by the combination of two simple salts in a fixed molar ratio that crystallizes together.
- On dissolving in water, it completely dissociates into its constituent ions.
- Mohr’s salt provides a stable source of ferrous (\( Fe^{2+} \)) ions for experiments.
Apparatus and Chemicals Required
To successfully perform the preparation of Mohr’s salt practical (as per Class 12 and NCERT guidelines), gather the following:
- Beaker, conical flask, funnel, china dish, glass rod
- Wire gauze, tripod stand, filter paper
- Weighing balance
- Ferrous sulphate heptahydrate (\( FeSO_4 \cdot 7H_2O \))
- Ammonium sulphate (\( (NH_4)_2SO_4 \))
- Distilled water
- Dilute sulphuric acid (to prevent hydrolysis)
Step-by-Step Procedure for Preparation of Mohr’s Salt
Follow this precise method for a successful Mohr’s salt preparation and optimal crystal yield:
- Accurately weigh 7 g of ferrous sulphate and 3.5 g of ammonium sulphate.
- Place both salts in a clean beaker.
- Add about 2-3 mL of dilute sulphuric acid to the beaker. This acid is crucial to suppress hydrolysis of iron(II) ions.
- Pour roughly 200 mL hot distilled water over the salts, a little at a time, stirring to help dissolution.
- Continue to stir until all solids dissolve, then filter the clear solution if impurities are visible.
- Transfer the filtrate to a china dish and heat gently to concentrate the solution, avoiding overheating.
- Check for crystallization by dipping a glass rod; crystals should form upon cooling.
- Once concentrated, let the solution cool undisturbed at room temperature for several hours or overnight.
- Carefully decant the mother liquor and collect pale green Mohr’s salt crystals using filter paper.
- Wash the crystals briefly with cold water and a small amount of alcohol (1:1 mixture), then dry them between filter papers.
Balanced Chemical Equation
The reaction involved in Mohr’s salt preparation is:
$$ FeSO_4 \cdot 7H_2O + (NH_4)_2SO_4 + 6H_2O + \text{dil.}~H_2SO_4 \rightarrow (NH_4)_2Fe(SO_4)_2 \cdot 6H_2O $$
Important Precautions During Preparation
- Always add dilute acid to prevent hydrolysis of ferrous ions, ensuring clarity and purity of crystals.
- Avoid overheating during solution concentration, as this can oxidize Fe2+ to Fe3+ and reduce yield.
- Let the solution cool slowly and undisturbed, which is essential for forming large, pure crystals.
- Minimize washing the final crystals, as Mohr’s salt is soluble and can be lost through excessive rinsing.
Physical Appearance and Properties
- Crystal Colour: Pale green
- Crystal Shape: Monoclinic
- Solubility: Highly soluble in water
- Litmus Test: Turns blue litmus red, confirming acidic nature
With this explanation, you are now equipped with the proper knowledge and technique for the preparation of Mohr’s salt in the lab environment. This practical is frequently asked about in preparation of Mohr’s salt viva questions and is a cornerstone experiment in preparation of Mohr’s salt ncert and Class 12 chemistry. Understanding this also supports further studies relating to titrations, analysis of potash alum, and other double salts.
To deepen your chemistry knowledge, you may also want to explore related topics such as crystal structures, chemistry lab practices, and melting point measurement for additional context.
FAQs on Preparation of Mohrs Salt in Laboratory with Principle and Reaction
1. What is Mohr’s salt and what is its chemical formula?
Mohr’s salt is a double salt of ferrous sulphate and ammonium sulphate with the chemical formula (NH4)2Fe(SO4)2·6H2O. It contains one Fe2+ ion, two NH4+ ions, and two SO42- ions along with six molecules of water of crystallization. It is also called ferrous ammonium sulphate and is commonly used in redox titrations.
2. How is Mohr’s salt prepared in the laboratory?
Mohr’s salt is prepared by dissolving ferrous sulphate and ammonium sulphate in dilute sulphuric acid followed by crystallization.
- Dissolve FeSO4·7H2O and (NH4)2SO4 separately in minimum distilled water containing a few drops of dilute H2SO4.
- Mix the two solutions and heat gently to obtain a clear solution.
- Filter if necessary and allow the solution to cool undisturbed.
- Green crystals of (NH4)2Fe(SO4)2·6H2O separate out on cooling.
3. Why is dilute sulphuric acid added during the preparation of Mohr’s salt?
Dilute sulphuric acid is added to prevent the oxidation of Fe2+ to Fe3+ during preparation.
- Fe2+ ions are easily oxidized by air to Fe3+.
- An acidic medium suppresses hydrolysis and oxidation.
- This helps in obtaining pure green crystals of Mohr’s salt.
4. What type of salt is Mohr’s salt?
Mohr’s salt is a double salt because it contains two simple salts that crystallize together in a fixed ratio.
- It is formed from FeSO4 and (NH4)2SO4.
- In aqueous solution, it dissociates into Fe2+, NH4+, and SO42- ions.
- It does not form complex ions in solution.
5. What is the balanced chemical reaction for the preparation of Mohr’s salt?
Mohr’s salt is formed by crystallization of ferrous sulphate and ammonium sulphate in solution as shown: FeSO4·7H2O(aq) + (NH4)2SO4(aq) → (NH4)2Fe(SO4)2·6H2O(s) + H2O(l).
- The reaction represents combination and crystallization.
- One molecule of water is released during crystal formation.
6. What are the properties of Mohr’s salt crystals?
Mohr’s salt forms light green, crystalline solids that are stable in air compared to ferrous sulphate.
- Color: Pale green crystals.
- Solubility: Soluble in water.
- Contains six molecules of water of crystallization.
- More resistant to oxidation than FeSO4 alone.
7. Why is Mohr’s salt considered more stable than ferrous sulphate?
Mohr’s salt is more stable because the presence of ammonium sulphate and acidic conditions reduce oxidation of Fe2+ to Fe3+.
- Ferrous sulphate alone oxidizes easily in air.
- The double salt structure stabilizes Fe2+ ions.
- It shows less tendency to form basic ferric salts.
8. What are the uses of Mohr’s salt in chemistry?
Mohr’s salt is mainly used as a primary standard in redox titrations.
- Used in titration with KMnO4 to determine oxidizing agents.
- Provides a stable source of Fe2+ ions.
- Used in analytical chemistry for standardization of solutions.
9. What precautions should be taken during the preparation of Mohr’s salt?
Mohr’s salt preparation requires maintaining acidic conditions and avoiding oxidation.
- Add a few drops of dilute H2SO4 to prevent Fe2+ oxidation.
- Use freshly prepared ferrous sulphate solution.
- Avoid excessive heating.
- Allow slow cooling for proper crystal formation.
10. How do you test the presence of Fe2+ in Mohr’s salt?
The presence of Fe2+ in Mohr’s salt can be confirmed by adding potassium ferricyanide, which forms a blue precipitate.
- Add K3[Fe(CN)6] solution to the salt solution.
- A blue precipitate (Turnbull’s blue) indicates Fe2+ ions.
