Section-A: Inorganic Chemistry

I(a). Inorganic Preparation and Reactions

Inorganic preparations involve synthesizing an inorganic compound from starting materials. Key steps are: reaction, filtration, crystallization, washing, and drying. The final yield and purity (color, crystal shape) are important.

Preparation of Chrome alum (Potassium chromium(III) sulfate)

Formula: KCr(SO4)2·12H2O

Principle: Potassium dichromate (an oxidizing agent) is reduced by ethanol in the presence of sulfuric acid. The Cr(VI) in dichromate is reduced to Cr(III). The resulting potassium sulfate and chromium(III) sulfate combine to form the double salt, chrome alum, which is crystallized from the solution.

Reaction:
K2Cr2O7 + 4 H2SO4 → K2SO4 + Cr2(SO4)3 + 4 H2O + 3 [O]
[ C2H5OH + [O] → CH3CHO + H2O ] x 3
Overall: K2Cr2O7 + 4 H2SO4 + 3 C2H5OH → K2SO4 + Cr2(SO4)3 + 3 CH3CHO + 7 H2O
K2SO4 + Cr2(SO4)3 + 24 H2O → 2 [KCr(SO4)2·12H2O]

Procedure (General Steps)

1. Dissolve potassium dichromate (K2Cr2O7) in water and add concentrated sulfuric acid (H2SO4) slowly with constant cooling and stirring. The solution will turn orange/red.
2. Slowly add ethanol (C2H5OH) drop by drop to this solution. This is a highly exothermic reaction and produces acetaldehyde (pungent smell). Keep the flask in an ice bath to control the temperature.
3. The solution will turn dark, greenish-black as Cr(VI) is reduced to Cr(III).
4. Allow the reaction to complete, then gently warm the solution if necessary to ensure all dichromate has reacted.
5. Cool the solution undisturbed in an ice bath to allow crystallization. Deep purple, octahedral crystals of chrome alum will form.
6. Filter the crystals, wash them with a small amount of ice-cold water (alum is soluble in water), and then with ethanol.
7. Dry the crystals on filter paper.

Preparation of copper (II) sulphate (Blue Vitriol)

Formula: CuSO4·5H2O

Principle: Copper metal does not react with dilute sulfuric acid. It is first oxidized by concentrated nitric acid to copper(II) oxide (or nitrate), which then readily dissolves in dilute sulfuric acid. A safer and more common lab method is to use copper(II) oxide or copper(II) carbonate directly.

Reaction (from Copper(II) Oxide):
CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)
CuSO4 (aq) + 5 H2O (l) --(Crystallization)--> CuSO4·5H2O (s)

Procedure (from CuO)

1. Gently warm dilute sulfuric acid (H2SO4) in a beaker.
2. Add copper(II) oxide (CuO), a black powder, in small portions (spatula-by-spatula) with stirring.
3. Keep adding CuO until some of it remains undissolved at the bottom. This ensures all the acid has been neutralized.
4. Filter the hot solution to remove the excess, unreacted CuO. The filtrate should be a clear, blue solution of copper(II) sulfate.
5. Transfer the filtrate to an evaporating dish and heat it gently to concentrate the solution (to about one-third of its original volume).
6. Allow the solution to cool slowly and undisturbed. Large, blue, triclinic crystals of copper(II) sulfate pentahydrate will form.
7. Decant the mother liquor (remaining solution), filter the crystals, and dry them.

Preparation of Sodium Trisoxalatochromate (III)

Formula: Na3[Cr(C2O4)3]·xH2O

Principle: A coordination complex is formed. Cr(III) ions are complexed by oxalate ligands (C2O4)2-, which act as bidentate ligands. The synthesis starts with the reduction of Cr(VI) (from dichromate) by oxalic acid. Oxalic acid acts as both the reducing agent and the ligand.

Reaction (Simplified):
K2Cr2O7 + 7 H2C2O4 + Na2C2O4 → 2 Na[Cr(C2O4)2(H2O)2] + K2C2O4 + 7 H2O + 6 CO2
Na[Cr(C2O4)2(H2O)2] + Na2C2O4 → Na3[Cr(C2O4)3] + 2 H2O

This preparation is complex and often done in stages. A common route involves mixing sodium dichromate and oxalic acid, leading to a vigorous reaction. The final product is crystallized as dark green crystals.

Preparation of Aluminium potassium sulphate (Potash alum)

Formula: KAl(SO4)2·12H2O

Principle: A double salt is formed by combining equimolar quantities of potassium sulfate (K2SO4) and aluminium sulfate (Al2(SO4)3). The compound crystallizes with 12 molecules of water.

Reaction:
K2SO4 (aq) + Al2(SO4)3 (aq) + 24 H2O (l) → 2 [KAl(SO4)2·12H2O] (s)

Procedure

1. Calculate the molar masses and weigh out equimolar amounts of K2SO4 and Al2(SO4)3·18H2O.
2. Dissolve both salts in a minimum amount of warm distilled water containing a few drops of dilute H2SO4 (to prevent hydrolysis of Al(III)).
3. If the solution is cloudy, filter it.
4. Mix the two clear solutions and concentrate the mixture by heating gently.
5. Cool the concentrated solution in an ice bath. Colorless, octahedral crystals of potash alum will precipitate.
6. Filter the crystals, wash with a small amount of ice-cold water, and dry.

Preparation of Tetramminecopper (II) phosphate

Formula: [Cu(NH3)4]3(PO4)2

Principle: This is the synthesis of a coordination complex. Copper(II) ions (from copper sulfate) are complexed by four ammonia (NH3) ligands to form the deep blue tetraamminecopper(II) complex cation, [Cu(NH3)4]²⁺. This cation is then precipitated as a phosphate salt by adding a source of phosphate ions (e.g., diammonium phosphate, (NH4)2HPO4).

Reaction:
CuSO4 + 4 NH3 → [Cu(NH3)4]SO4 (Schweizer's reagent)
3 [Cu(NH3)4]SO4 + 2 (NH4)2HPO4 → [Cu(NH3)4]3(PO4)2 (s) + 3 (NH4)2SO4

The product is a characteristic blue-violet precipitate.

I(b). Titrimetric Analysis

Titrimetric analysis is a quantitative method used to determine the concentration of an unknown solution. This section focuses on the foundational skills required: calibrating equipment and preparing solutions.

Calibration of Glassware

Purpose: Volumetric glassware (pipettes, burettes, volumetric flasks) is manufactured to hold or deliver a specific volume, but there are always small errors. Calibration is the process of checking the *actual* volume held or delivered by the glassware.

Principle of Calibration

The method is gravimetric. We measure the mass of a pure liquid (distilled water) contained or delivered by the glassware. Using the known density of water at a specific temperature, we can convert this mass into the true volume.

Formula: Volume (V) = Mass (m) / Density (ρ)

Procedure (e.g., Calibrating a 25 mL Volumetric Flask)

1. Clean and dry the 25 mL volumetric flask thoroughly.
2. Weigh the empty, dry flask accurately on an analytical balance (Mass 1, m1).
3. Fill the flask with distilled water exactly to the calibration mark (bottom of the meniscus on the line). Use a dropper for the last few drops.
4. Measure the temperature of the water.
5. Weigh the flask filled with water (Mass 2, m2).
6. Calculate the mass of the water: m = m2 - m1.
7. Look up the density of water (ρ) at the measured temperature. (e.g., at 25°C, ρ ≈ 0.9970 g/mL).
8. Calculate the true volume: V_true = m / ρ.
9. Compare V_true to the nominal volume (25.00 mL) to find the calibration error.

For Pipettes/Burettes: You weigh a clean, dry beaker (m1), deliver the specified volume of water from the burette/pipette into it, and weigh it again (m2). The calculation is the same.

Preparation of Solutions of different Molarity / Normality

Definitions

• Molarity (M): Moles of solute per liter of solution.
  Formula: Molarity = (Moles of solute) / (Volume of solution in L)
  Moles = (Mass in g) / (Molar mass in g/mol)
• Normality (N): Gram equivalent weight of solute per liter of solution.
  Formula: Normality = (Gram equivalents) / (Volume of solution in L)
  Normality = Molarity × n-factor
• n-factor (valency factor):
  • For acids: Number of replaceable H⁺ ions (e.g., H2SO4, n=2; HCl, n=1).
  • For bases: Number of replaceable OH⁻ ions (e.g., NaOH, n=1; Ca(OH)2, n=2).
  • For redox agents: Number of electrons lost or gained per molecule (e.g., KMnO4 in acidic medium, MnO4⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O, so n=5).

Procedure (e.g., Prepare 250 mL of 0.1 M Oxalic Acid)

1. Identify the Solute: Oxalic acid dihydrate (H2C2O4·2H2O).

2. Calculate Molar Mass:
(12.01×2) + (1.01×2) + (16.00×4) + 2×(1.01×2 + 16.00) = 24.02 + 2.02 + 64.00 + 36.04 = 126.08 g/mol

3. Calculate Required Moles:
Molarity = Moles / Volume (L)
0.1 M = Moles / 0.250 L
Moles = 0.1 × 0.250 = 0.025 mol

4. Calculate Required Mass:
Mass = Moles × Molar Mass
Mass = 0.025 mol × 126.08 g/mol = 3.152 g

5. Prepare the Solution:

1. Weigh out exactly 3.152 g of oxalic acid dihydrate on a watch glass using an analytical balance.
2. Carefully transfer the solid to a 250 mL volumetric flask using a funnel.
3. Wash the watch glass and funnel with a small amount of distilled water, letting the washings run into the flask (to ensure all solid is transferred).
4. Add distilled water to the flask until it is about half full. Swirl to dissolve the solid completely.
5. Once dissolved, carefully add more distilled water until the bottom of the meniscus touches the 250 mL calibration mark.
6. Stopper the flask and invert it 15-20 times to ensure the solution is homogeneous.