Preparation of Mohr’s Salt or Ferrous Ammonium Sulphate

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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) sulfate. 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 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 crystallized, 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) cation 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 crystallization 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:

Color of the crystals 

Light Green 

Shape of the crystals 

Monoclinic shaped 

Solubility in water 

Soluble 

Blue litmus paper (Litmus test)

Blue litmus paper turns red

Result:

Yield of Mohr’s salt or ferrous ammonium sulphate is ________g.

Precautions: 

  • For crystallization, the solution should cool down slowly. 

  • Do not disturb the solution while cooling during crystallization. 

  • Don’t overheat the solution for crystallization. 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 diarrhea 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.