Mohrs Salt - Formula, Structure, Preparation, Properties, FAQs

What is Mohr salt?

Mohr's salt is a double salt of ferrous sulfate and ammonium sulfate whose chemical formula is $\left(\mathrm{NH}_4\right)_2 \mathrm{Fe}\left(\mathrm{SO}_4\right)_2\left(\mathrm{H}_2 \mathrm{O}\right)_6$. Mohr's salt appears as pale green crystals and is an important source of Fe²⁺ (ferrous) ions in laboratory work. It is more stable than ordinary ferrous sulfate because the ammonium sulfate present in the crystal structure prevents the oxidation of Fe²⁺ to Fe³⁺ by atmospheric oxygen. Due to its high purity and stability, Mohr's salt is widely used as a primary standard in redox titrations, particularly those involving potassium permanganate (KMnO₄) and potassium dichromate ($\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7$). It is highly soluble in water and contains six molecules of water of crystallisation.

Structure of Mohr salt

• Mohr's salt has the chemical formula $\left(\mathrm{NH}_4\right)_2 \mathrm{Fe}\left(\mathrm{SO}_4\right)_2 \cdot 6 \mathrm{H}_2 \mathrm{O}$ and is a double salt composed of one ferrous sulfate $\left(\mathrm{FeSO}_4\right)$ molecule and one ammonium sulfate $\left(\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4\right)$ molecule with six water molecules of crystallization.
• The $\mathrm{Fe}^{2+}$ ion is octahedrally coordinated by six water molecules, forming the complex ion $\left[\mathrm{Fe}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{2+}$, which gives the crystal its characteristic pale green colour.
• The sulfate ions $\left(\mathrm{SO}_4{ }^{2-}\right)$ and ammonium ions $\left(\mathrm{NH}_4{ }^{+}\right)$are present as separate ions in the crystal lattice and are held together by strong electrostatic forces and hydrogen bonding.
• The six water molecules of crystallisation help stabilise the crystal structure and protect the $\mathrm{Fe}^{2+}$ ion from oxidation, making Mohr's salt more stable than ordinary ferrous sulfate.

Preparation of Mohr's Salt

Mohr's salt is prepared by dissolving ferrous sulfate $\left(\mathrm{FeSO}_4 \cdot 7 \mathrm{H}_2 \mathrm{O}\right)$ and ammonium sulfate $\left(\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4\right)$ in water containing a small amount of dilute sulfuric acid. The acidic medium prevents the oxidation of Fe²⁺ ions to Fe³⁺. The solution is heated until both salts dissolve completely and then allowed to cool slowly. On cooling, pale green crystals of Mohr's salt separate out. These crystals are filtered, washed with a small amount of cold water or alcohol, and dried.

$\mathrm{FeSO}_4 \cdot 7 \mathrm{H}_2 \mathrm{O}+\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4 \rightarrow\left(\mathrm{NH}_4\right)_2 \mathrm{Fe}\left(\mathrm{SO}_4\right)_2 \cdot 6 \mathrm{H}_2 \mathrm{O}+\mathrm{H}_2 \mathrm{O}$

Related Topics Link -

• Coordination Number
• Calcium Carbonate
• Calcium Oxide

Properties of Mohr's Salt

• Mohr's salt has the formula $\left(\mathrm{NH}_4\right)_2 \mathrm{Fe}\left(\mathrm{SO}_4\right)_2 \cdot 6 \mathrm{H}_2 \mathrm{O}$ and is a double salt of ferrous sulfate and ammonium sulfate.
• It occurs as pale green or light green crystalline solid due to the presence of $\mathrm{Fe}^{2+}$ ions.
• It is highly soluble in water but practically insoluble in alcohol.
• Mohr's salt is more stable than ferrous sulfate because ammonium sulfate reduces the oxidation of $\mathrm{Fe}^{2+}$ to $\mathrm{Fe}^{3+}$ in air.
• It contains six molecules of water of crystallization, which help maintain its crystal structure.
• The $\mathrm{Fe}^{2+}$ ion is surrounded by six water molecules in an octahedral arrangement, while NH₄⁺ and SO₄^2- ions remain outside the coordination sphere.
• Its aqueous solution is slightly acidic, which further helps prevent oxidation of ferrous ions.
• Due to its high purity and stability, it is widely used as a primary standard in redox titrations involving potassium permanganate $\left(\mathrm{KMnO}_4\right)$ and potassium dichromate $\left(\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7\right)$.

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Some Solved Examples

Question 1: The equivalent weight of Mohr's salt, $\left(\mathrm{NH}_4\right)_2 \mathrm{Fe}\left(\mathrm{SO}_4\right)_2 \cdot 6 \mathrm{H}_2 \mathrm{O}$, in acidic $\mathrm{KMnO}_4$ titration is:
(A) $392 \mathrm{~g} \mathrm{eq}^{-1}$
(B) $196 \mathrm{~g} \mathrm{eq}^{-1}$
(C) $98 \mathrm{~g} \mathrm{eq}^{-1}$
(D) $56 \mathrm{~g} \mathrm{eq}^{-1}$

Solution:

The molar mass of Mohr's salt is:

$392 \mathrm{~g} \mathrm{~mol}^{-1}$
One mole of Mohr's salt contains one $\mathrm{Fe}^{2+}$ ion.

$\mathrm{Fe}^{2+} \rightarrow \mathrm{Fe}^{3+}+e^{-}$
Since one mole loses one mole of electrons, the $\mathbf{n}$-factor $=\mathbf{1}$.

$\text { Equivalent weight }=\frac{\text { Molar mass }}{n \text {-factor }}=\frac{392}{1}=392 \mathrm{~g} \mathrm{eq}^{-1}$

Hence, the correct answer is option (A).

Question 2: The percentage by mass of iron (Fe) in Mohr's salt, $\left(\mathrm{NH}_4\right)_2 \mathrm{Fe}\left(\mathrm{SO}_4\right)_2 \cdot 6 \mathrm{H}_2 \mathrm{O}$, is closest to:
(A) $10.2 %$
(B) $14.3 %$
(C) $18.4 %$
(D) $22.8 %$

Solution:

Molar mass of Mohr's salt:

$392 \mathrm{~g} \mathrm{~mol}^{-1}$
Mass of Fe per mole:

$56 \mathrm{~g}$
Percentage of iron:

$\frac{56}{392} \times 100=14.29 \% \approx 14.3 %$

Hence, the correct answer is option (B).

Question 3: Mohr's salt, $\left(\mathrm{NH}_4\right)_2 \mathrm{Fe}\left(\mathrm{SO}_4\right)_2 \cdot 6 \mathrm{H}_2 \mathrm{O}$, is widely used as a primary standard in redox titrations. Which one of the following statements correctly explains this property?
(A) It contains $\mathrm{Fe}^{3+}$ ions that are highly stable in air.
(B) The presence of ammonium sulfate stabilizes $\mathrm{Fe}^{2+}$ ions, making the salt less susceptible to oxidation by atmospheric oxygen.
(C) It is completely insoluble in water, preventing oxidation of $\mathrm{Fe}^{2+}$ ions.
(D) It acts as a strong oxidizing agent in acidic medium.

Solution:

Mohr's salt is a double salt of ferrous sulfate and ammonium sulfate:

$\left(\mathrm{NH}_4\right)_2 \mathrm{Fe}\left(\mathrm{SO}_4\right)_2 \cdot 6 \mathrm{H}_2 \mathrm{O}$
The $\mathbf{F e}^{\mathbf{2 +}}$ ions present in Mohr's salt are more stable than those in ordinary ferrous sulfate because ammonium sulfate reduces their oxidation to $\mathrm{Fe}^{3+}$ in the presence of air. This high stability, along with its purity, makes Mohr's salt an excellent primary standard for redox titrations such as $\mathrm{KMnO}_4$ and $\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7$ titrations.

Hence, the correct answer is option (B).

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