Theory & Procedure, Preparation of Inorganic Compounds | Additional Study Material for NEET PDF Download

Objectives

Our objectives are to prepare:

• A pure sample of ferrous ammonium sulphate (Mohr’s salt) [FeSO₄. (NH₄)₂SO₄. 6H₂O]
• A pure sample of potash alum (Fitkari) [K₂SO₄. Al₂(SO₄)₃. 24H₂O]
• A pure sample of the complex potassium trioxalatoferrate (III), K₃[Fe(C₂O₄)₃]. 3H₂O

The Theory

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⁺, N^a+, 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.

Learning Outcomes

• Students understand the terms ‘inorganic compounds, double salts and coordination compounds.
• Students will acquire the skills to prepare  Mohr’s salt , Potash alum  and  Potassium trioxalatoferrate
• Students will identify the chemicals and apparatus required for the preparation of Mohr’s salt, potash alum and potassium trioxalatoferrate.
• Once students understand the concept of the experiment and the different steps, they can perform the experiment in the real lab more accurately and quickly.

A. Preparation of pure sample of Ferrous ammonium sulphate (Mohr’s salt) [FeSO₄.(NH₄)₂SO₄.6H₂O]

Materials Required

• Ferrous sulphate
• Ammonium sulphate
• Dil. Sulphuric acid
• Ethyl alcohol
• Distilled water
• Beakers
• China dish
• Funnel
• Glass rod
• Tripod stand
• Wire gauze
• Burner
• Wash bottle 
• Measuring jar
• Electronic balance

Procedure

Real Lab Procedure

1. We’ll first take 7g ferrous sulphate 3.5g ammonium sulphate in a clean 250ml beaker.
2. To this add about 2-3ml of dil.sulphuric acid to prevent the hydrolysis of ferrous sulphate.
3. In another beaker, boil about 20ml of water for 5 minutes.
4. Add the boiling hot water to the contents in the first beaker in small quantities at a time.
5. Stir the contents of the beaker with a glass rod until the salts have completely dissolved.
6. Filter the solution into a china dish.
7. Now heat the solution in the china dish until its crystallisation point is reached. Then transfer the solution into a crystallising dish and keep it undisturbed.
8. On cooling, crystals of Mohr’s salt separate.
9. Decant the mother liquor and wash the crystals with a small quantity of alcohol and then dry the crystals by placing them between filter paper pads.
10. Find the weight of the crystals.

Observations

1. Weight of the crystals obtained = ....g
2.  Colour of the crystals = ....
3.  Shape of the crystals = ....

B. Preparation of pure sample of potash alum (Fitkari) [K₂SO₄.Al₂(SO₄)₃.24H₂O]

Materials Required

• Potassium sulphate
• Aluminium sulphate
• Dil. Sulphuric acid
• Distilled water
• Beakers
• China dish
• Funnel
• Glass rod
• Tripod stand
• Wire gauze
• Burner
• Wash bottle
• Measuring jar
• Electronic balance

Procedure

Real Lab 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 = ....g
2. Colour of the crystals = ....
3. Shape of the crystals = ....

C. Preparation of the complex Potassium trioxalatoferrate (III) {K₃[Fe(C₂O₄)₃].3H₂O}

Materials Required

• Ferric chloride
• Hydrated oxalic acid
• Hydrated potassium oxalate
• Potassium hydroxide
• Distilled water
• Beakers
• China dish
• Funnel
• Glass rod
• Tripod stand
• Wire gauze
• Burner
• Wash bottle 
• Measuring jar
• Electronic balance

Procedure

Real Lab 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 = ....g
2. Colour of the crystals = ....

Simulator Procedure (as performed through the Online Labs)

You can select a compound from ‘Select the compound’ drop down list.

Ferrous ammonium sulphate

• Drag the watch glass to the beaker to transfer 7 g ferrous sulphate into it.
• Drag the watch glass to the beaker to add 3.5 g ammonium sulphate to the beaker containing ferrous sulphate.
• Drag the measuring jar to the beaker to pour 3 ml dil. H₂SO₄ into it.
• Click on the “Next” button to go to the next process.
• Drag the beaker to pour 20 ml hot water into the beaker containing ferrous sulphate and ammonium sulphate.
• Drag the glass rod to the beaker to stir the contents of the beaker.
• Drag the beaker to the funnel to filter the solution into the china dish.
• Click on the “Next” button to go to the next process.
• Click on the knob of the burner to turn it on.
• Drag the glass rod to the china dish to stir the solution.
• Drag the china dish to the crystallising dish to transfer the solution.
• Drag the watch glass to the crystallising dish to cover the dish with it.
• Drag the crystallising dish to the cold water trough to place the dish in it.
• Wait for some time to cool the solution.
• Click on the “Next” button to go to the next process.
• Drag the crystallising dish to the beaker to decant the mother liquor into it.
• Drag the bottle to the crystallising dish to pour ethyl alcohol into it.
• Drag the crystallising dish to shake it and wash the crystals with alcohol.
• Drag the crystallising dish to the filter paper to transfer the crystals.
• Drag the other filter paper to the filter paper containing crystals to dry the crystals.
• Drag the filter paper to the watch glass to transfer the crystals into it.
• Click on the inference icon to see the inference.
• To redo the experiment, click on the ‘Reset’ button.

Note: Click on the ‘HELP’ button to see the instructions.

 Potash alum

• Drag the watch glass to the beaker to add 2.5 g potassium sulphate into 20 ml distilled water.
• Drag the watch glass to the beaker to add 10 g aluminium sulphate into 20 ml distilled water.
• Drag the measuring jar to the beaker to pour 1 ml dil. dil. H₂SO₄ into it.
• Click on the “Next” button to go to the next process.
• Drag the beaker to the burner to place it over the wire gauze and heat aluminium sulphate solution.
• Drag the beaker to the china dish to transfer aluminium sulphate solution into it.
• Drag the beaker containing potassium sulphate solution to the china dish to transfer the solution into it.
• Click on the “Next” button to go to the next process.
• Click on the knob of the burner to turn it on.
• Drag the glass rod to the china dish to stir the solution.
• Drag the china dish to the crystallising dish to transfer the solution.
• Drag the watch glass to the crystallising dish to cover the dish with it.
• Drag the crystallising dish to the cold water trough to place the dish in it.
• Wait for some time to cool the solution.
• Click on the “Next” button to go to the next process.
• Drag the crystallising dish to the beaker to decant the mother liquor into it.
• Drag the bottle to the crystallising dish to pour ethyl alcohol-cold water mixture into it.
• Drag the crystallising dish to shake it and wash the crystals with alcohol-cold water mixture.
• Drag the crystallising dish to the filter paper to transfer the crystals into it.
• Drag the other filter paper to the filter paper containing crystals to dry the crystals.
• Drag the filter paper to the watch glass to transfer the crystals into it.
• Click on the inference icon to see the inference.
• To redo the experiment, click on the ‘Reset’ button.

Note: Click on the ‘HELP’ button to see the instructions.

Potassium trioxalatoferrate (III)

• Drag the watch glass to the beaker to add 3.5 g ferric chloride into 50 ml distilled water.
• Drag the watch glass to the beaker to add 4 g potassium hydroxide into 50 ml distilled water.
• Drag the glass rod to the beaker containing potassium hydroxide to stir the contents of the beaker.
• Drag the beaker to add potassium hydroxide solution to the beaker containing ferric chloride solution.
• Click on the inference icon to see the inference.
• Click on the “Next” button to go to the next process.
• Drag the filter paper to the Buchner funnel to place it in the funnel.
• Drag the beaker to the Buchner funnel to filter the contents of the beaker.
• Click on the inference icon to see the inference.
• Click on the “Next” button to go to the next process.
• Drag the watch glass to the beaker to add 4 g hydrated oxalic acid into 100 ml distilled water.
• Drag the watch glass to the beaker to add 5.5 g hydrated potassium oxalate to the beaker.
• Drag the watch glass to the beaker to add ferric hydroxide precipitate into the solution of potassium hydroxide and oxalic acid.
• Drag the beaker to the funnel to filter the solution into the china dish.
• Click on the “Next” button to go to the next process.
• Click on the knob of the burner to turn it on.
• Drag the glass rod to the china dish to stir the solution.
• Drag the china dish to the cold water trough to place the dish in it.
• Drag the black paper to the cold water trough to cover the china dish with it.
• Wait for some time to cool the solution in the china dish.
• Click on the “Next” button to go to the next process.
• Drag the bottle to the china dish to pour ethyl alcohol into it.
• Drag the china dish to shake it and wash the crystals with ethyl alcohol.
• Drag the china dish to the filter paper to transfer the crystals into it.
• Drag the other filter paper to the filter paper containing crystals to dry the crystals.
• Drag the filter paper to the watch glass to transfer the crystals into it.
• Click on the inference icon to see the inference.
• To redo the experiment, click on the ‘Reset’ button.

Note: Click on the ‘HELP’ button to see the instructions.

Precautions

1. Do not concentrate the solution too much.
2. To get large crystals, let the concentrated solution cool slowly and remain undisturbed.
3. Do not heat the solution of ferrous sulphate and ammonium sulphate for a long time as it may oxidise ferrous ions to ferric ions.