The Aluminium Carbonate Formula is written as \ ( \text{Al}_2(\text{CO}_3)_3 \), representing a carbonate salt of aluminium that is highly unstable under standard conditions. This compound is covered in NCERT Chemistry for Classes 11 and 12, particularly in chapters dealing with s-block and p-block elements, salts, and chemical equilibrium. Students preparing for JEE Main, JEE Advanced, and NEET frequently encounter questions on aluminium compounds and their behaviour. This article covers the formula, structure, properties, a complete chemistry formula sheet, three solved examples, CBSE exam tips, common mistakes, and JEE/NEET applications.
Key Aluminium Carbonate Formulas at a Glance
Quick reference for the most important formulas related to aluminium carbonate.
- Aluminium Carbonate: \( \text{Al}_2(\text{CO}_3)_3 \)
- Molar Mass: \( M = 2(27) + 3(12 + 48) = 234 \ \text{g/mol} \)
- Decomposition: \( \text{Al}_2(\text{CO}_3)_3 \rightarrow \text{Al}_2\text{O}_3 + 3\text{CO}_2 \)
- Formation from double displacement: \( \text{Al}_2(\text{SO}_4)_3 + 3\text{Na}_2\text{CO}_3 \rightarrow \text{Al}_2(\text{CO}_3)_3 + 3\text{Na}_2\text{SO}_4 \)
- Hydrolysis in water: \( \text{Al}_2(\text{CO}_3)_3 + 3\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 + 3\text{CO}_2 \)
- Aluminium hydroxide formula: \( \text{Al(OH)}_3 \)
- Aluminium oxide formula: \( \text{Al}_2\text{O}_3 \)
What is Aluminium Carbonate Formula?
The Aluminium Carbonate Formula, \( \text{Al}_2(\text{CO}_3)_3 \), represents the theoretical carbonate salt formed when aluminium ions combine with carbonate ions. Aluminium carries a 3+ charge, while the carbonate ion \( \text{CO}_3^{2-} \) carries a 2− charge. To balance these charges, two aluminium ions and three carbonate ions combine. This gives the formula \( \text{Al}_2(\text{CO}_3)_3 \).
It is important to note that aluminium carbonate does not exist stably under normal atmospheric conditions. When formed, it immediately decomposes into aluminium hydroxide and carbon dioxide in the presence of water, or into aluminium oxide and carbon dioxide upon heating. This instability makes it chemically significant. Despite its instability, the concept of its formula is tested in NCERT Class 11 Chemistry (Chapter 10 — The s-Block Elements and Chapter 11 — The p-Block Elements) and Class 12 Chemistry. Understanding this formula helps students grasp ionic bonding, charge balancing, and the chemistry of aluminium salts.
The compound has a theoretical molar mass of 234 g/mol. Its IUPAC name is dialuminium tricarbonate. The compound is white in colour in its theoretical solid form.
Aluminium Carbonate Formula — Expression and Variables
\[ \text{Al}_2(\text{CO}_3)_3 \]
The formula is derived by balancing the charges of the aluminium cation and the carbonate anion. Aluminium has a valency of 3 (Al³⁺). The carbonate ion has a charge of 2− (CO₃²⁻). Using the criss-cross method, the subscripts become 2 for Al and 3 for CO₃.
| Symbol | Element / Ion | Charge / Valency | Count in Formula |
|---|---|---|---|
| Al | Aluminium | 3+ | 2 |
| C | Carbon (in carbonate) | — | 3 |
| O | Oxygen (in carbonate) | — | 9 |
| CO₃²⁻ | Carbonate ion | 2− | 3 |
| M | Molar Mass | — | 234 g/mol |
Derivation of the Formula
The formula is derived using the ionic charge balance method.
Step 1: Identify the ions. Aluminium forms Al³⁺. Carbonate is CO₃²⁻.
Step 2: Apply the criss-cross rule. The charge of Al (3) becomes the subscript of CO₃. The charge of CO₃ (2) becomes the subscript of Al.
Step 3: Write the formula: \( \text{Al}_2(\text{CO}_3)_3 \).
Step 4: Verify charge balance. Total positive charge = \( 2 \times 3 = +6 \). Total negative charge = \( 3 \times 2 = -6 \). The charges balance. The formula is correct.
Molar Mass Calculation:
\[ M = 2 \times 27 + 3 \times (12 + 3 \times 16) = 54 + 3 \times 60 = 54 + 180 = 234 \ \text{g/mol} \]
Complete Chemistry Formula Sheet — Aluminium Compounds and Carbonates
| Formula Name | Expression | Variables / Notes | Molar Mass | NCERT Chapter |
|---|---|---|---|---|
| Aluminium Carbonate | \( \text{Al}_2(\text{CO}_3)_3 \) | Unstable; decomposes in water | 234 g/mol | Class 11, Ch 11 |
| Aluminium Hydroxide | \( \text{Al(OH)}_3 \) | Amphoteric hydroxide | 78 g/mol | Class 11, Ch 11 |
| Aluminium Oxide | \( \text{Al}_2\text{O}_3 \) | Amphoteric oxide; corundum | 102 g/mol | Class 11, Ch 11 |
| Aluminium Sulphate | \( \text{Al}_2(\text{SO}_4)_3 \) | Used in water purification | 342 g/mol | Class 11, Ch 11 |
| Aluminium Chloride | \( \text{AlCl}_3 \) | Lewis acid; used in Friedel-Crafts | 133.5 g/mol | Class 11, Ch 11 |
| Sodium Carbonate | \( \text{Na}_2\text{CO}_3 \) | Washing soda; basic salt | 106 g/mol | Class 11, Ch 10 |
| Calcium Carbonate | \( \text{CaCO}_3 \) | Limestone; decomposes on heating | 100 g/mol | Class 11, Ch 10 |
| Carbonate Ion | \( \text{CO}_3^{2-} \) | Charge = 2−; planar structure | 60 g/mol | Class 11, Ch 11 |
| Decomposition of Al₂(CO₃)₃ | \( \text{Al}_2(\text{CO}_3)_3 \rightarrow \text{Al}_2\text{O}_3 + 3\text{CO}_2 \) | Thermal decomposition | — | Class 11, Ch 11 |
| Hydrolysis of Al₂(CO₃)₃ | \( \text{Al}_2(\text{CO}_3)_3 + 3\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 + 3\text{CO}_2 \) | Occurs immediately in water | — | Class 12, Ch 7 |
| Alum Formula | \( \text{KAl(SO}_4)_2 \cdot 12\text{H}_2\text{O} \) | Potassium alum; double salt | 474 g/mol | Class 12, Ch 7 |
Aluminium Carbonate Formula — Solved Examples
Example 1 (Class 9-10 Level) — Writing and Balancing the Formula
Problem: Write the chemical formula of aluminium carbonate and calculate its molar mass. (Atomic masses: Al = 27, C = 12, O = 16)
Given: Aluminium ion = Al³⁺, Carbonate ion = CO₃²⁻
Step 1: Identify the charges. Al has a charge of 3+. CO₃ has a charge of 2−.
Step 2: Apply the criss-cross rule. Subscript of Al = 2 (from the charge of CO₃). Subscript of CO₃ = 3 (from the charge of Al).
Step 3: Write the formula: \( \text{Al}_2(\text{CO}_3)_3 \).
Step 4: Calculate molar mass.
\[ M = 2(27) + 3(12 + 3 \times 16) = 54 + 3(60) = 54 + 180 = 234 \ \text{g/mol} \]
Answer
The Aluminium Carbonate Formula is \( \text{Al}_2(\text{CO}_3)_3 \) and its molar mass is 234 g/mol.
Example 2 (Class 11-12 Level) — Hydrolysis Reaction and Mass Calculation
Problem: 23.4 g of aluminium carbonate undergoes complete hydrolysis in water. Find the mass of aluminium hydroxide produced.
Given: Mass of Al₂(CO₃)₃ = 23.4 g, Molar mass of Al₂(CO₃)₃ = 234 g/mol, Molar mass of Al(OH)₃ = 78 g/mol
Step 1: Write the hydrolysis equation.
\[ \text{Al}_2(\text{CO}_3)_3 + 3\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 + 3\text{CO}_2 \]
Step 2: Calculate moles of Al₂(CO₃)₃.
\[ n = \frac{23.4}{234} = 0.1 \ \text{mol} \]
Step 3: Use stoichiometry. 1 mol of Al₂(CO₃)₃ produces 2 mol of Al(OH)₃. So 0.1 mol produces \( 0.1 \times 2 = 0.2 \) mol of Al(OH)₃.
Step 4: Calculate mass of Al(OH)₃.
\[ m = 0.2 \times 78 = 15.6 \ \text{g} \]
Answer
The mass of aluminium hydroxide produced is 15.6 g.
Example 3 (JEE/NEET Level) — Volume of CO₂ Produced at STP
Problem: 46.8 g of aluminium carbonate is heated strongly until complete decomposition. Calculate the volume of CO₂ gas produced at STP. (Molar volume at STP = 22.4 L/mol)
Given: Mass of Al₂(CO₃)₃ = 46.8 g, Molar mass = 234 g/mol
Step 1: Write the thermal decomposition equation.
\[ \text{Al}_2(\text{CO}_3)_3 \xrightarrow{\Delta} \text{Al}_2\text{O}_3 + 3\text{CO}_2 \]
Step 2: Calculate moles of Al₂(CO₃)₃.
\[ n_{\text{Al}_2(\text{CO}_3)_3} = \frac{46.8}{234} = 0.2 \ \text{mol} \]
Step 3: Find moles of CO₂. From stoichiometry, 1 mol of Al₂(CO₃)₃ gives 3 mol of CO₂.
\[ n_{\text{CO}_2} = 0.2 \times 3 = 0.6 \ \text{mol} \]
Step 4: Calculate volume at STP.
\[ V = 0.6 \times 22.4 = 13.44 \ \text{L} \]
Answer
The volume of CO₂ produced at STP is 13.44 litres.
CBSE Exam Tips 2025-26
- Memorise the formula correctly: Always write \( \text{Al}_2(\text{CO}_3)_3 \) with the correct subscripts. A common error is writing AlCO₃. We recommend practising the criss-cross method at least five times.
- Remember the instability: CBSE frequently asks why aluminium carbonate does not exist under normal conditions. The answer is that it immediately hydrolyses in the presence of moisture to give Al(OH)₃ and CO₂.
- Learn both decomposition reactions: Thermal decomposition gives Al₂O₃ and CO₂. Hydrolysis in water gives Al(OH)₃ and CO₂. Both are important for 2025-26 board exams.
- Molar mass calculations are scoring: In stoichiometry questions, always write the molar mass of each compound first. This avoids arithmetic errors and earns step marks.
- Amphoteric nature of aluminium compounds: CBSE 2025-26 papers often test the amphoteric nature of Al(OH)₃ and Al₂O₃. Connect this to the aluminium carbonate hydrolysis product.
- Use NCERT examples: Our experts suggest revising NCERT Class 11 Chapter 11 (The p-Block Elements) thoroughly. Aluminium compounds appear in at least one question every year.
Common Mistakes to Avoid
- Wrong formula subscripts: Many students write \( \text{AlCO}_3 \) or \( \text{Al}_3(\text{CO}_3)_2 \). Always apply the criss-cross rule carefully. The correct formula is \( \text{Al}_2(\text{CO}_3)_3 \).
- Confusing carbonate with bicarbonate: Aluminium carbonate is \( \text{Al}_2(\text{CO}_3)_3 \). Aluminium bicarbonate would be \( \text{Al(HCO}_3)_3 \). These are different compounds with different formulas.
- Incorrect molar mass: Students often forget to multiply the oxygen atoms correctly. The carbonate ion CO₃²⁻ has a mass of 60 g/mol (12 + 48). With three carbonate groups, the contribution is 180 g/mol, not 60 g/mol.
- Wrong decomposition products: Thermal decomposition gives Al₂O₃ (not Al(OH)₃). Hydrolysis gives Al(OH)₃ (not Al₂O₃). Mixing these up is a very common mistake in exams.
- Stating it exists stably: A conceptual error is claiming that aluminium carbonate is a stable compound. It is not. Always mention its instability and immediate decomposition in the presence of water or heat.
JEE/NEET Application of Aluminium Carbonate Formula
In our experience, JEE aspirants encounter aluminium carbonate primarily in the context of p-block chemistry, stoichiometry, and ionic equilibrium. NEET aspirants see it in the context of antacids and biological chemistry. Here are the key application patterns.
Pattern 1 — Stoichiometry and Mole Concept (JEE Main)
JEE Main frequently includes stoichiometry problems involving the decomposition of carbonates. A question may give the mass of Al₂(CO₃)₃ and ask for the volume of CO₂ at STP or NTP. The key steps are: calculate moles using molar mass 234 g/mol, apply the 1:3 mole ratio from the decomposition equation, and multiply by 22.4 L/mol (STP) or 24 L/mol (NTP, 1 bar).
Pattern 2 — Ionic Charge Balancing (NEET and JEE Main)
Both JEE and NEET test the ability to write correct ionic formulas. Questions ask students to identify the correct formula among options. The trick is knowing that Al has a 3+ charge and CO₃ has a 2− charge. The correct formula is always \( \text{Al}_2(\text{CO}_3)_3 \). Incorrect options often include AlCO₃, Al₂CO₃, or Al₃(CO₃)₂.
Pattern 3 — Antacid Chemistry (NEET Biology and Chemistry)
NEET tests the medicinal use of aluminium compounds. Aluminium carbonate acts as an antacid. It neutralises excess hydrochloric acid in the stomach. The reaction is:
\[ \text{Al}_2(\text{CO}_3)_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O} + 3\text{CO}_2 \]
It also binds phosphate in the intestines. This is tested in NEET under human physiology and medicinal chemistry. In our experience, at least one question per year in NEET relates to the action of antacids at the molecular level.
Pattern 4 — Percentage Composition (JEE Advanced)
JEE Advanced may ask for the percentage of aluminium or oxygen in \( \text{Al}_2(\text{CO}_3)_3 \). The calculation uses:
\[ \% \text{Al} = \frac{2 \times 27}{234} \times 100 = \frac{54}{234} \times 100 \approx 23.08\% \]
\[ \% \text{O} = \frac{9 \times 16}{234} \times 100 = \frac{144}{234} \times 100 \approx 61.54\% \]
FAQs on Aluminium Carbonate Formula
Explore more related chemistry formulas on ncertbooks.net. Visit our Chemistry Formulas hub for a complete list of all important formulas. You may also find these articles useful: Bleaching Powder Formula, Silicon Dioxide Formula, and Ammonia Formula. For official NCERT content, refer to the NCERT official website.