Introduction
Mohr salt is a double swab containing ferrous sulphate and ammonium sulphate in equimolar quantities. Hence, when these two mariners are mixed in the rate of their molecular millions and also a hot logged result prepared, filtered and the hot filtrate cooled, light blue or green chargers of Mohr swab (octahedral in shape) are attained.
Mohr’s swab is prepared by dissolving an equimolar admixture of doused FeSO4.7H2Oand ammonium sulphate in the water containing a little sulphuric acid. Take a clean 250 ml teacup; transfer 7 g of FeSO4 and 3.5 g of ammonium sulphate chargers to it. Add 2.3 ml of dilute sulphuric acid to help hydrolysis. 200 ml of water is boiled for 5 twinkles. Add boiling hot water to the contents in small amounts. Now stir until mariners have fully dissolved. Sludge it and toast the filtrate in a demitasse dish till it concentrates to the Crystallisation Point. Place the demitasse dish for cooling for about 12 hours and collect the chargers of Mohr’s swab using the sludge paper.
Compared the attained Mohr’s swab and original Mohr’s swab, using chemical styles similar as permanganometry and dichrometry system. Compare the attained Mohr’s swab and original Mohr’s swab using physical styles similar to FTIR spectroscopy, UV-Visible spectroscopy and melting point determination.
Mohr’s salt is an inorganic compound which is a double salt with the formula (NH4)2Fe(SO4)2(H2O)6. It is a common laboratory reagent with IUPAC name ammonium iron(II) sulphate. Mohr’s salt is named after German Chemist Karl Fredrich Mohr.
Aim:
To prepare double salt ferrous ammonium sulphate or Mohr’s salt.
Theory:
A double salt is a salt that contains an equimolar mixture of two salts with different cations but the same anion. Mohr’s salt is a double salt. So, when equimolar proportions of ferrous sulphate and ammonium sulphate are mixed to form a solution and then crystallised, we get crystals of ferrous ammonium salt or Mohr’s salt. The reaction can be shown as follows –
FeSO4 + (NH4)2SO4 + 6H2O 🡪 FeSO4.(NH4)2SO4.6H2O
Ferrous Ammonium Water Ferrous Ammonium Sulphate
Sulphate Sulphate or Mohr’s Salt
Iron(II) cations can undergo hydrolysis, therefore to prevent it while preparing their aqueous solution, a small amount of dilute sulphuric acid is added.
Requirements:
Beaker, conical flask, trough, glass rod, tripod stand, funnel, wire gauze, weighing machine, ferrous sulphate, ammonium sulphate, distilled water, dilute sulphuric acid.
Procedure:
Weigh 3.5 g of ferrous sulphate and keep it separately.
Weigh 1.7 g of ammonium sulphate and keep it separately.
Take a conical flask of 50ml and wash it with distilled water.
Now take 5ml of distilled water in the conical flask and add earlier weighed quantities of ferrous sulphate and ammonium sulphate.
Add 0.5ml of dilute sulphuric acid in the conical flask to prevent hydrolysis of iron cations.
Now gently warm the mixture (solution) to get a clear solution.
Filter the solution.
Concentrate the solution(filtrate) by heating it till crystallisation point is reached.
Stir the solution occasionally while heating.
Allow the mixture to cool to room temperature.
After cooling the solution, you will get crystals of ferrous ammonium sulphate (green colored).
Decant the mother liquor and separate the crystals.
Now wash the crystals with water and alcohol mixture (1:1).
Dry the crystals and note down the yield of the product.
Observation Table:
Result:
Yield of Mohr’s salt or ferrous ammonium sulphate is ________g.
Precautions:
For crystallisation, the solution should cool down slowly.
Do not disturb the solution while cooling during crystallisation.
Don’t overheat the solution for crystallisation. As it may change ferrous ions into ferric ions which will change stoichiometry of the solution.
Mohr’s salt is largely used in analytical chemistry. Inhalation of Mohr’s salt causes irritation in the nose and throat. Over exposure of it can cause diarrhoea as well. So, while experimenting you need to be very careful.
This was all about preparation of Mohr’s salt, if you are looking for Mohr’s salt titrations which are important lab experiments of Class XII chemistry then you can go through the page ‘Mohr’s salt titration against potassium permanganate’. Many such articles are available on Vedantu to help you. If you also want to get NCERT Solutions of Chemistry Class XII then register yourself on Vedantu or download Vedantu learning app for class 6-10, IIT-JEE and NEET.
FAQs on Preparation of Mohr’s Salt or Ferrous Ammonium Sulphate
1. What is Mohr's salt and what is its chemical formula?
Mohr's salt is a crystalline solid with a light green colour. It is a double salt with the chemical formula (NH₄)₂Fe(SO₄)₂·6H₂O. Its official IUPAC name is ammonium iron(II) sulphate hexahydrate. It is commonly used in laboratories as a stable source of ferrous ions (Fe²⁺).
2. What is the principle behind the preparation of Mohr's salt?
The preparation of Mohr's salt is based on the principle of crystallisation from a solution containing equimolar amounts of its constituent salts. When hydrated ferrous sulphate (FeSO₄·7H₂O) and ammonium sulphate ((NH₄)₂SO₄) are dissolved together in water and the solution is concentrated, the less soluble double salt, Mohr's salt, crystallises out upon cooling. The chemical reaction is:
FeSO₄ + (NH₄)₂SO₄ + 6H₂O → (NH₄)₂Fe(SO₄)₂·6H₂O
3. How is Mohr's salt prepared in the laboratory as per the Class 12 practical syllabus?
To prepare Mohr's salt in a lab, you need to follow these steps:
- Take equimolar quantities of hydrated ferrous sulphate and ammonium sulphate in a beaker.
- Dissolve the salts in a minimum amount of distilled water containing a few drops of dilute sulphuric acid to prevent hydrolysis.
- Gently warm the solution to get a clear solution.
- Filter the solution to remove any insoluble impurities.
- Concentrate the filtrate by heating it in a china dish until the crystallisation point is reached. You can check this by dipping a glass rod and seeing if a solid crust forms upon cooling.
- Allow the concentrated solution to cool slowly and undisturbed.
- After several hours, light green crystals of Mohr's salt will separate out.
- Decant the mother liquor, wash the crystals with a small amount of cold water, and dry them between filter paper sheets.
4. What are the main applications or uses of Mohr's salt?
Mohr's salt is highly valued in analytical chemistry for several reasons:
- It serves as a reliable primary standard in volumetric analysis, particularly in titrations involving potassium permanganate (permanganometry) and potassium dichromate (dichrometry).
- It is a stable source of ferrous ions (Fe²⁺). Unlike ferrous sulphate, which can be easily oxidised by air to ferric ions (Fe³⁺), Mohr's salt is much more resistant to oxidation, ensuring greater accuracy in experiments.
- It is used in various quantitative analyses to determine the concentration of oxidising agents.
5. Why is dilute sulphuric acid added during the preparation of Mohr's salt?
Dilute sulphuric acid is crucial for preventing the hydrolysis of ferrous sulphate. In an aqueous solution, ferrous ions (Fe²⁺) can react with water to form iron(II) hydroxide, Fe(OH)₂, which is a greenish precipitate. This reaction would consume the ferrous ions needed to form Mohr's salt. Adding acid increases the H⁺ ion concentration, which, according to Le Chatelier's principle, suppresses the hydrolysis reaction and keeps the Fe²⁺ ions dissolved in the solution.
6. Is Mohr's salt different from ferrous ammonium sulphate?
No, they are the same compound. 'Mohr's salt' is the common name given to ferrous ammonium sulphate in honour of the German chemist Karl Friedrich Mohr, who pioneered its use in titrimetry. The correct chemical name is ammonium iron(II) sulphate hexahydrate.
7. Why is Mohr's salt considered a double salt and not a complex salt?
Mohr's salt is a double salt because it exists as a single stable crystalline compound in the solid state but completely dissociates into its constituent simple ions when dissolved in water. An aqueous solution of Mohr's salt contains Fe²⁺ (aq), NH₄⁺ (aq), and SO₄²⁻ (aq) ions. In contrast, a complex salt contains a complex ion that does not dissociate in solution. For example, K₄[Fe(CN)₆] dissolves to give K⁺ and the stable [Fe(CN)₆]⁴⁻ complex ion.
8. What key precautions must a student take while preparing crystals of Mohr's salt?
For a successful preparation and a good yield of pure crystals, a student should observe the following precautions:
- The solution should be cooled slowly and without any disturbance to allow the growth of large, well-defined crystals.
- Heating during concentration should be gentle and brief. Overheating or prolonged heating can oxidise the ferrous (Fe²⁺) ions to ferric (Fe³⁺) ions, resulting in a brownish, impure product.
- Always use a minimal amount of water for washing the final crystals, as Mohr's salt is soluble in water and excessive washing will reduce the yield.
- The initial addition of dilute H₂SO₄ is essential and should not be skipped, as it prevents the formation of precipitates due to hydrolysis.
