Materials and Procedure for Preparing Mohr’s Salt Solution
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 How to Prepare Mohr’s Salt in the Laboratory
1. What is Mohr's salt and how is it prepared in the laboratory?
Mohr's salt is a double salt of ferrous sulfate and ammonium sulfate with the formula (NH4)2SO4 · FeSO4 · 6H2O. It can be easily prepared in the school laboratory by dissolving calculated amounts of ferrous sulfate and ammonium sulfate in dilute sulfuric acid and allowing the solution to crystallize.
- Mix equimolar amounts of FeSO4 · 7H2O and (NH4)2SO4.
- Dissolve in minimum amount of dilute sulfuric acid to prevent hydrolysis.
- Filter the solution if necessary, then set aside to crystallize at room temperature.
2. What is the chemical formula and structure of Mohr's salt?
Mohr's salt has the chemical formula (NH4)2SO4 · FeSO4 · 6H2O.
- It is a double salt containing ferrous sulfate, ammonium sulfate, and six molecules of water of crystallization.
- It is stable, light green in color, and less prone to oxidation than plain ferrous sulfate.
3. Why is dilute sulfuric acid used in the preparation of Mohr's salt?
Dilute sulfuric acid is used to prevent oxidation of Fe2+ ions to Fe3+ and to inhibit hydrolysis of ferrous sulfate.
- Keeps iron in the +2 oxidation state.
- Ensures purity and stability of Mohr's salt crystals.
4. What are the steps involved in the preparation of Mohr's salt crystals?
The preparation of Mohr's salt involves these steps:
- Weigh required quantities of ferrous sulfate and ammonium sulfate.
- Dissolve separately in a minimum quantity of distilled water, both containing a few drops of dilute H2SO4.
- Mix both solutions in a beaker.
- Filter if necessary, then leave the solution undisturbed to allow slow crystallization.
- Collect, wash, and dry the green Mohr's salt crystals.
5. What are the uses and importance of Mohr's salt?
Mohr's salt is important in titrations and chemical analysis due to its stable composition:
- Used as a primary standard in redox titrations for estimating oxidizing agents (like KMnO4).
- Preferred because it does not oxidize easily in air.
- Also used in agriculture, water treatment, and as a laboratory reagent.
6. What precautions should be taken while preparing Mohr's salt?
To obtain pure Mohr's salt crystals, these precautions must be followed:
- Always use freshly prepared solutions to avoid oxidation of Fe2+.
- Use distilled water and clean apparatus.
- Add dilute H2SO4 to prevent hydrolysis and oxidation.
- Do not disturb the solution during crystallization.
7. What is the role of ammonium sulfate in Mohr's salt?
Ammonium sulfate forms a double salt with ferrous sulfate, providing stability to the compound.
- Decreases the rate of oxidation of Fe2+ ions.
- Facilitates the crystallization of Mohr's salt.
8. Why does Mohr's salt require six molecules of water of crystallization?
Six water molecules of crystallization stabilize the crystal lattice of Mohr's salt.
- They hold the constituents together in a definite ratio.
- Give the salt its unique green color and crystal shape.
9. State two differences between Mohr's salt and ferrous sulfate.
Mohr's salt differs from ferrous sulfate in these ways:
- Mohr's salt is a double salt of FeSO4 and (NH4)2SO4, while ferrous sulfate is a single salt.
- Mohr's salt is more stable and less prone to oxidation than plain ferrous sulfate.
10. What are the uses of dilute sulfuric acid in the preparation and storage of Mohr's salt?
Dilute sulfuric acid both prepares and preserves Mohr's salt by:
- Preventing oxidation of Fe2+ ions during crystallization and storage.
- Inhibiting hydrolysis that leads to decomposition of the salt.
- Maintaining clarity and stability for titration and experiments.
