How to Prepare Mohr's salt | Chemistry Project | PDF

Mohr's salt

Mohr's salt is an example of a ‘double salt’, i.e. it contains the same ions as in (FeSO4·7H2O) and
(NH4)2SO4. It is simple to prepare because it is less soluble than either of its constituent salts. A
double salt in solution displays the chemistries of its component ions because of no new chemical
bonds are formed.
The salt is named after the German chemist Karl Friedrich Mohr, who made many important
advances in the methodology of titration in the 19th century. It is preferred over iron(II) sulfate for
titrations as it is less prone to oxidation by air. The oxidation of iron(II) to iron(III) occurs more
rapidly at higher pH; Mohr’s Salt lowers the pH of solutions slightly.

Aim of the experiment

To prepare the Mohr's salt

Theory

Mohr’s salt chemically known as Ammonium iron(II) sulfate having the formula FeSO4(NH4)2SO4.6H2O has molar mass 392gmol-1. It is a primary standard. Its equivalent mass is 392/1 = 392 as its n factor is 1 as per the following reaction:
Fe2+ → Fe3+ + e-
It is a common laboratory reagent. Like the other ferrous sulfate salts, ferrous ammonium sulfate dissolves in water to give the aquo complex which has octahedral molecular geometry. Mohr’s salt appears as a blue-green solid. Its mineral form is mohrite.

Preparation of Mohr's salt

The method of preparing Mohr's salt involves the dissolving an equimolar mixture of hydrated ferrous sulfate (FeSO4·7H2O) and ammonium sulfate (NH4)2SO4 in water; containing a little of sulphuric acid and the crystallization from the solution. On crystallization light green crystals of ferrous ammonium; sulfate separates out.

Apparatus and Chemical Required

100 mL Beaker, Top-pan Balance, Stirrer Hotplate,  Magnetic Follower, Iron(II) Sulfate Heptahydrate (FeSO4.7H2O; 12.5g), Dilute(1 M) Sulfuric Acid (5 mL), Deionized Water (10 mL), Ammonium Sulfate [(NH4)2(SO4); 6g]

Steps involved in the preparation of Mohr's salt

Note: For this part of the experiment, you should work in a fume hood.
  1. Place a clean 100 mL beaker onto a top-pan balance, and set the display to read zero, this is known as taking the balance.
  2. Weigh directly into the beaker iron(II) sulfate heptahydrate (FeSO4.7H2O; 12.5g) from the reagent bottle. Remove the beaker from the balance and carefully add dilute(1 M) sulfuric acid (5 mL), and deionized water (10 mL). 
  3. Heat the beaker on a stirrer hotplate, stirring with a magnetic follower until all of the solid has dissolved.
  4. You should obtain a clear, pale green solution.
  5. Place a second clean 100 mL beaker onto a top-pan balance and take the balance. 
  6. Weigh directly into the beaker ammonium sulfate [(NH4)2(SO4); 6g]. 
  7. Remove the beaker from the balance and add deionized water (8 mL). 
  8. Heat the beaker on a stirrer hotplate, stirring with a magnetic follower until all of the solid has dissolved. 
  9. You should obtain a colorless solution. 
  10. Carefully pour the contents of the beaker containing the solution of iron(II) sulfate into the beaker containing the ammonium sulfate solution. 
  11. The combined volume should be approximately 30 mL.
  12. Heat the beaker to boiling until the initially opaque green solution becomes clear.
  13. Note: Do not let the beaker boil dry.
  14. The final solution should be a blue-green color. 
  15. You will isolate your product by crystallization to get the light green crystals of Mohr's salt.
  16. Cool your beaker first to room temperature, and then in an ice-water bath.

Results

Light Green crystals of Mohr's salt
Shape of the crystals: Monoclinic

Health Risks

Inhalation: Causes irritation to the respiratory tract symptoms may include coughing, shortness of breath
Ingestion: Causes irritation to the gastrointestinal tract. Symptoms may include nausea, vomiting, and diarrhea. Low toxicity in small quantities but larger dosages may cause nausea, vomiting, diarrhea and black stool. Pink urine discoloration is a strong indicator of iron poisoning. Liver damage, coma, and death from iron poisoning have been recorded.
Skin contact: Causes irritation to skin. Symptoms include redness itching and pain
Eye contact: Causes irritation, redness and pain
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