CBSE Class 12 chemistry practical · Preparation of compounds

Source: CBSE Class 12 chemistry practical syllabus · use your school laboratory manual (NCERT Lab Manual where your school uses it).

Preparation of Mohr’s salt (Class 12 chemistry practical)

You dissolve ferrous sulphate and ammonium sulphate in dilute acid, then crystallise Mohr’s salt.

Masses, acid volumes, and yield calculations come from your laboratory manual — enter what you actually used and obtained.

Published practice page. Concentrations, masses, and numerical answers come from your school laboratory manual and your own readings — not invented here. Not yet reviewed by a named chemistry reviewer. Version 1.0.0, updated 2026-10-03.

Safety — read before you start

Safety warning. Practise here. Perform the real experiment only in your school lab, with your teacher present.

Dilute sulphuric acid is corrosive.
Iron salts can irritate skin and eyes.
Hot solutions can scald — handle beakers carefully.

Wear: Lab coat, Splash goggles, Gloves, as your teacher directs.

Follow your teacher’s disposal instructions. Do not pour leftovers down a household drain.

Loads only after you tap. Use the lab hands to pick up and use equipment (pointer hidden in the scene). Falls back to a flat diagram if WebGL is unavailable.

Aim

To prepare crystals of Mohr’s salt (ferrous ammonium sulphate hexahydrate) from the reagents and quantities specified in your laboratory manual.

Methods and quantities can differ between schools. Follow your teacher's instructions.

Apparatus and chemicals

Apparatus and chemicals
Item	Note
Beakers	For dissolving and mixing.
Glass rod	Stirring.
Funnel and filter paper	If your manual requires filtration of hot solution.
China dish / crystallising dish	For crystallisation as directed.
Watch glass / filter paper	For drying crystals as your teacher shows.
Balance	Weighing reagents and product.
Ferrous sulphate heptahydrate (FeSO4·7H2O)	Use the concentration, mass, and volume values printed in your school laboratory manual. This page does not invent a numerical result for you.
Ammonium sulphate ((NH4)2SO4)	Use the concentration, mass, and volume values printed in your school laboratory manual. This page does not invent a numerical result for you.
Dilute sulphuric acid (H2SO4)	Small amount as in the manual — helps prevent Fe²⁺ oxidation and hydrolysis during preparation.
Distilled water (H2O)	For dissolving and washing as directed.

Theory and reaction

Open

Mohr’s salt is a double salt: FeSO₄·(NH₄)₂SO₄·6H₂O.

Equimolar (or manual-specified) amounts of FeSO₄·7H₂O and (NH₄)₂SO₄ dissolve in dilute H₂SO₄; on cooling, the double salt crystallises.

Dilute acid reduces oxidation of Fe²⁺ to Fe³⁺ and formation of basic salts.

Ferrous sulphate heptahydrate plus ammonium sulphate gives Mohr’s salt hexahydrate plus water.

Picture guide

Crystals forming in a beaker during salt preparation — Crystals grow as the hot saturated solution cools. Filter, wash, and dry as your manual says — do not invent a mass here.

Procedure

Weigh the masses of FeSO₄·7H₂O and (NH₄)₂SO₄ your manual states.
Dissolve them in the minimum volume of distilled water containing dilute H₂SO₄ as directed.Safety: Dilute sulphuric acid is corrosive.
Warm gently if the manual says so; filter hot if instructed to remove undissolved matter.
Cool slowly for crystallisation. Decant mother liquor carefully.
Wash crystals briefly as your teacher shows, dry between filter papers, and weigh the yield.

Observation table

Enter the readings from your school lab in the practical file below. This page does not fill them in for you.

Type the readings in your practical file

S. No.	Mass of FeSO₄·7H₂O (g)	Mass of (NH₄)₂SO₄ (g)	Mass of crystals obtained (g)
1
2
3

Calculation

Theoretical yield uses molar masses and the limiting reagent from your weighed amounts. Percentage yield = (actual / theoretical) × 100. Use only your weighed values.

Result

Report the mass of Mohr’s salt obtained and, if required, the percentage yield.

Precautions

Keep a little dilute H₂SO₄ present as the manual describes.
Do not boil vigorously if the manual only asks for gentle warming.
Avoid prolonged heating in air, which can oxidise Fe²⁺.
Dry crystals without overheating.

Viva questions

Answer one question at a time. The full answers stay closed until you open them.

Question 1 of 3

Mohr’s salt is best described as…

A. A complex salt onlyB. A double salt of FeSO₄ and (NH₄)₂SO₄C. An oxide of ironD. Pure ammonium chloride

Pick an answer first.

Show all questions and answers

Mohr’s salt is best described as…
Answer: A double salt of FeSO₄ and (NH₄)₂SO₄. Reason: It crystallises as FeSO₄·(NH₄)₂SO₄·6H₂O.
Why is dilute H₂SO₄ added during preparation?
Answer: To reduce oxidation / hydrolysis of Fe²⁺. Reason: Acidic medium helps keep iron in the Fe²⁺ state during crystallisation.
How many water molecules are in the usual Mohr’s salt formula used in school?
Answer: 6. Reason: The hexahydrate has six water molecules of crystallisation.

Common mistakes

Omitting dilute acid so rusty Fe³⁺ compounds form.
Using tap water when the manual specifies distilled water.
Reporting a copied yield instead of the crystals you weighed.

Frequently asked questions

Can I do this at home?: No. Use this page to practise the order of steps and viva. Do the real experiment only in your school laboratory, with your teacher present.

Your practical file

Name, school, and roll number are optional and are not sent anywhere.

The practical file opens on this device.

Review

Not yet reviewed by a named chemistry reviewer. Published for practice only — follow your teacher and laboratory manual for quantities and safety.

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Related notes

Mole, formula, and concentration