Preparation of Double Salt
* Mohrs Salt
* Potash Alum
What is an inorganic compound?
Inorganic compounds are substances that do not come from living things. They are formed by non-living natural processes or by laboratory preparation methods. The branch of chemistry that deals with the behaviour and properties of inorganic compounds is called Inorganic Chemistry. Inorganic compounds are found in nature in the form of minerals.
What are the different types of inorganic compounds?
The two important classes of inorganic compounds are Coordination Compounds and Double Salts.
Coordination compounds (complex compounds)
Complex compounds are formed by a large number of transition metals in which the metal atom is bound to neutral molecules or to negatively charged species called ligands. The elements of group 3-12 are called transition metals.
These compounds are also called coordination compounds. The ligands donate electrons to the metal atoms and the metal atoms accept these electrons to form a ligand-metal coordinate bond. The number of ligands directly bonded to the central metal atom is called the coordination number of a complex. The structure of coordination compounds was first proposed by Alfred Werner. He proposed the concept of a primary valence and secondary valence for a metal ion. Primary valences are satisfied by the central ions and secondary valences are satisfied by the ligands. Secondary valence is equal to the coordination number.
On example is Potassium trioxalatoferrate (III) { K3[Fe(C2O4)3].3H2O}
Potassium trioxalatoferrate (III) is a coordination compound. In this complex, iron is the central metal ion and oxalate [(C2O4)2-] is the ligand. Oxalate is a bidentate ligand in which two oxygen atoms donate electrons to the central iron atom. It is an octahedral transition metal complex in which iron is in the +3 oxidation state. So we can say that in complex potassium trioxalatoferrate (III), the central Fe3+ ion is octahedrally surrounded by bidentate oxalate ligands. Potassium acts as the counter ion, and the Fe3+ ion, along with the ligand, constitute the coordination sphere. The structure is shown below.
Other examples are [Co(NH3)6]3+, [PtCl4]2-, Ni(CO)4.
In coordination compounds, the central metal ion and the ligands attached to it are enclosed in a square bracket and are collectively termed as the coordination sphere. The ionisible group is written outside the bracket and is called counter ion. For example, in the complex, K2[PdCl4], the coordination sphere is [PdCl4]2- and the counter ion is K+.
Double Salts
Double salts are compounds that contain more than one cation or anion. Double salts are obtained by the combination of two different salts. The two salts crystallise together to form a single substance, but it ionises as two distinct salts when dissolved in water. The properties of double salts are different from its component single salts.
One example is Ferrous ammonium sulphate (Mohr’s salt) [FeSO4.(NH4)2SO4.6H2O]
Ferrous ammonium sulphate is a double salt of ferrous sulphate and ammonium sulphate (Mohr’s salt). It has the formula FeSO4.(NH4)2SO4.6H2O. It contains two different cations Fe2+ and NH4+. It is used as a primary standard in volumetric analysis. The crystals are light green coloured and are monoclinic in shape.
Another example is Alum. Alums are the most common examples of double salts. These are double sulphates having general formula X2SO4.M2(SO4)3.24H2O
Where X = monovalent cation such as K+, Na+, NH4+ etc.
M = trivalent cation such as Al3+, Fe3+,Cr3+ etc.
If you take Potash alum (Fitkari) [K2SO4.Al2(SO4)3.24H2O], it is a double salt of potassium sulphate and aluminium sulphate. The crystals of potash alum are octahedral in shape and are highly soluble in water. It is commonly used in the purification of water.
Other examples are Soda alum (Na2SO4.Al2(SO4)3.24H2O), Ferric alum (NH4)2SO4. Fe2(SO4)3. 24H2O) etc.
These are crystalline solids and are soluble in water. Due to hydrolysis, their aqueous solutions have acidic character.
Class 12 Chemistry Practical
EXPERIMENT NO. 1
Aim: To prepare the pure sample of ferrous sulphate (Mohr’s salt) FeSO4. (NH4)2SO4.6H2O.
Theory:- Mohr’s salt is a double salt of ferrous sulphate and ammonium sulphate. It is prepared by dissolving equimolar amount of hydrated FeSO4 and (NH4)2SO4 in water containing little of sulphuric acid. The resulting solution is then subjected to crystallisation when light green crystals of Mohr’s salt is separated out.
FeSO4.7H2O + (NH4)2SO4 -------------> FeSO4. (NH4)2SO4. 6H2O + H2O
(278 gms) (132 gms) (Mohr’s salt)
Procedure:-
1. Take 7 gm of ferrous sulphate and 3.5 gm ammonium sulphate in a clean 250 ml beaker and
add 2-3 ml dilute H2SO4 to prevent hydrolysis of ferrous sulphate into it.
2. In another 250 ml beaker, boil 20-25 ml water for 5 minutes to expel dissolved O2.
3. Pour the boiling water to the contents of first beaker in small instalments. Stir the mixture with glass rod till the salts have completely dissolved.
4. Filter the contents and collect the filtrate in china dish.
5. Concentrate the solution of china dish by heating up to the crystallization point.
6. Place the china dish over a beaker full of cold water. On cooling crystals of Mohr’s salt are separated out.
7. Decant off the mother liquor and wash the crystals with ethyl alcoholto remove any sulphuric acid sticking to the crystals.
8. Dry the crystals by placing them between the folds of filter paper.
9. Weigh the crystals and record the yield.
Result:-
1. Yield of crystals = ............gms
2. Colour of crystals = ............
3. Shape of crystals = ............
Precautions:-
1. Add 2-3 ml of dilute H2SO4 to prevent hydrolysis of FeSO4.
2. Concentrate the solution only till crystallisation point.
3. Do not disturb the solution during cooling.
EXPERIMENT NO. 2
Aim: Preparation of pure sample of potash alum [K2SO4.Al2(SO4)3.24H2O]
Procedure:
1. Take 2.5g potassium sulphate crystals in a clean beaker.
2. To this add 20ml of distilled water and stir using a glass rod until the crystals completely dissolve.
3. Take 10g aluminium sulphate in another beaker.
4. Add about 20ml of distilled water and 1ml of dil.sulphuric acid to it.
5. Heat the contents of the beaker for about 5 minutes.
6. Mix the two solutions in a china dish.
7. Heat the solution in the china dish for some time to concentrate it to the crystallisation point.
8. Transfer the solution into a crystallising dish and do not disturb it.
9. On cooling crystals of potash alum separate.
10. Decant the mother liquor and wash the crystals with a small quantity of ice-cold water.
11. Dry the crystals by placing them between filter paper pads.
12. Find the weight of the crystals.
Observations
1. Weight of the crystals obtained = ............ gms
2. Colour of the crystals = ..............
3. Shape of the crystals = ...............
EXPERIMENT NO. 3
Aim: Preparation of the complex Potassium trioxalatoferrate (III) {K3[Fe(C2O4)3].3H2O}
Procedure:
1. Take 3.5g of anhydrous ferric chloride in
a250ml beaker and dissolve it in 50ml water.
2. In another beaker dissolve 4g of potassium hydroxide in 50ml water.
3. Add potassium hydroxide to ferric chloride solution in small proportions while stirring constantly.
4. Filter the precipitate of ferric hydroxide formed through a Buchner funnel and wash it with distilled water.
5. In another beaker, take 4g of hydrated oxalic acid and 5.5g of hydrated potassium oxalate.
6. Add about 100ml of water and stir thoroughly to get a clear solution.
7. Add the freshly prepared ferric hydroxide precipitate in small amounts to the above solution while stirring constantly until the precipitate dissolves.
8. Filter the above solution into a china dish and heat over a sand bath until its crystallisation point is reached.
9. Place the china dish in a beaker full of cold water and keep it aside for filtration.
10. Cover the china dish with a piece of black paper because the complex is sensitive to light.
11. Decant the mother liquor, wash the crystals with a small amount of ethyl alcohol and dry them between the folds of filter paper.
12. Find the weight of the crystals.
Observations
1. Weight of the crystals obtained = ............ gms
2. Colour of the crystals = ............
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