The Aluminium Sulfide Formula is represented as Al₂S₃, formed by the ionic combination of aluminium (Al) and sulfide (S²⁻) ions. This compound is a key topic in NCERT Class 11 Chemistry, Chapter 10 (The s-Block Elements) and Chapter 3 (Classification of Elements). It also appears in CBSE Class 10 Science under chemical reactions. JEE Main and NEET aspirants frequently encounter Al₂S₃ in questions on ionic compounds, molar mass calculations, and hydrolysis reactions. This article covers the formula, properties, derivation, a complete formula sheet, three solved examples, and expert exam tips.
Key Aluminium Sulfide Formulas at a Glance
Quick reference for the most important formulas related to aluminium sulfide.
- Molecular formula: \( \text{Al}_2\text{S}_3 \)
- Molar mass: \( M = 2(27) + 3(32) = 150 \text{ g/mol} \)
- Hydrolysis reaction: \( \text{Al}_2\text{S}_3 + 6\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 + 3\text{H}_2\text{S} \)
- Formation reaction: \( 2\text{Al} + 3\text{S} \rightarrow \text{Al}_2\text{S}_3 \)
- Number of moles: \( n = \frac{m}{M} \)
- Density: \( \rho = 2.02 \text{ g/cm}^3 \)
- Melting point: \( 1100^\circ\text{C} \)
What is Aluminium Sulfide Formula?
The Aluminium Sulfide Formula, Al₂S₃, represents an inorganic ionic compound formed between aluminium (Al³⁺) cations and sulfide (S²⁻) anions. Aluminium carries a +3 oxidation state, while sulfur carries a −2 oxidation state in this compound. To achieve electrical neutrality, two aluminium ions combine with three sulfide ions, giving the formula Al₂S₃.
This compound appears as a yellow to grey crystalline solid at room temperature. It has a molar mass of 150.16 g/mol. Its melting point is 1100 °C and its boiling point is approximately 1500 °C. The density of Al₂S₃ is 2.02 g/cm³.
In NCERT Class 11 Chemistry, this compound is discussed in the context of ionic bonding and the chemistry of aluminium (Group 13 elements, Chapter 11). CBSE Class 10 students encounter it while studying chemical formulae and ionic compounds. Understanding Al₂S₃ is essential for grasping concepts of cross-multiplication in formula writing, hydrolysis reactions, and stoichiometry — all of which are tested in board exams and JEE/NEET.
Aluminium Sulfide Formula — Expression and Variables
The molecular formula of aluminium sulfide is written using the criss-cross method of ionic charges:
\[ \text{Al}_2\text{S}_3 \]
Aluminium has a valency of +3 and sulfur has a valency of −2. Using the criss-cross rule, the numerical value of aluminium’s charge (3) becomes the subscript of sulfur, and the numerical value of sulfur’s charge (2) becomes the subscript of aluminium. This gives Al₂S₃.
| Symbol | Element / Quantity | Value / SI Unit |
|---|---|---|
| Al | Aluminium (cation) | Oxidation state: +3 |
| S | Sulfur (anion) | Oxidation state: −2 |
| M | Molar mass of Al₂S₃ | 150.16 g/mol |
| ρ | Density | 2.02 g/cm³ |
| Tm | Melting point | 1100 °C |
| Tb | Boiling point | ~1500 °C |
| n | Number of moles | mol |
Derivation of the Aluminium Sulfide Formula
We derive the formula Al₂S₃ using the criss-cross method of ionic charges.
Step 1: Identify the ions. Aluminium forms Al³⁺ and sulfur forms S²⁻.
Step 2: Write the symbols side by side: Al S.
Step 3: Apply the criss-cross rule. The charge of Al (3) becomes the subscript of S. The charge of S (2) becomes the subscript of Al.
Step 4: Write the formula: \( \text{Al}_2\text{S}_3 \).
Step 5: Verify electrical neutrality: \( 2 \times (+3) + 3 \times (-2) = +6 – 6 = 0 \). The formula is electrically neutral and correct.
The molar mass is calculated as: \( M = 2 \times 27 + 3 \times 32 = 54 + 96 = 150 \text{ g/mol} \).
Complete Chemistry Formula Sheet — Aluminium and Sulfur Compounds
| Formula Name | Expression | Key Variables | SI Units / Values | NCERT Chapter |
|---|---|---|---|---|
| Aluminium Sulfide | \( \text{Al}_2\text{S}_3 \) | Al: +3, S: −2 | Molar mass: 150.16 g/mol | Class 11, Ch 11 |
| Aluminium Oxide | \( \text{Al}_2\text{O}_3 \) | Al: +3, O: −2 | Molar mass: 101.96 g/mol | Class 11, Ch 11 |
| Aluminium Chloride | \( \text{AlCl}_3 \) | Al: +3, Cl: −1 | Molar mass: 133.34 g/mol | Class 11, Ch 11 |
| Aluminium Hydroxide | \( \text{Al(OH)}_3 \) | Al: +3, OH: −1 | Molar mass: 78.00 g/mol | Class 11, Ch 11 |
| Aluminium Sulfate | \( \text{Al}_2( ext{SO}_4)_3 \) | Al: +3, SO₄: −2 | Molar mass: 342.15 g/mol | Class 11, Ch 11 |
| Hydrolysis of Al₂S₃ | \( \text{Al}_2\text{S}_3 + 6\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 + 3\text{H}_2\text{S} \) | Reactants: Al₂S₃, H₂O | Products: Al(OH)₃, H₂S | Class 11, Ch 11 |
| Formation of Al₂S₃ | \( 2\text{Al} + 3\text{S} \rightarrow \text{Al}_2\text{S}_3 \) | Al metal + sulfur | Exothermic reaction | Class 11, Ch 11 |
| Molar Mass Formula | \( M = \sum (\text{atomic mass} \times \text{subscript}) \) | M = molar mass | g/mol | Class 9, Ch 3 |
| Number of Moles | \( n = \frac{m}{M} \) | m = mass (g), M = molar mass | mol | Class 9, Ch 3 |
| Hydrogen Sulfide | \( \text{H}_2\text{S} \) | H: +1, S: −2 | Molar mass: 34.08 g/mol | Class 11, Ch 16 |
Aluminium Sulfide Formula — Solved Examples
Example 1 (Class 9-10 Level) — Writing the Formula
Problem: Write the chemical formula of aluminium sulfide. Verify that the formula is electrically neutral.
Given: Aluminium ion: Al³⁺ (charge = +3), Sulfide ion: S²⁻ (charge = −2)
Step 1: Write the symbols of the ions: Al³⁺ and S²⁻.
Step 2: Apply the criss-cross rule. The charge of Al (3) becomes the subscript of S. The charge of S (2) becomes the subscript of Al.
Step 3: Write the formula: \( \text{Al}_2\text{S}_3 \).
Step 4: Verify electrical neutrality: \( 2 \times (+3) + 3 \times (-2) = +6 – 6 = 0 \). The net charge is zero.
Answer
The chemical formula of aluminium sulfide is Al₂S₃. The formula is electrically neutral (net charge = 0).
Example 2 (Class 11-12 Level) — Molar Mass and Moles Calculation
Problem: Calculate the molar mass of aluminium sulfide (Al₂S₃). Then find the number of moles present in 300.32 g of the compound. (Atomic mass: Al = 27 g/mol, S = 32 g/mol)
Given: Formula = Al₂S₃, mass (m) = 300.32 g, Atomic mass of Al = 27 g/mol, Atomic mass of S = 32 g/mol
Step 1: Calculate the molar mass using \( M = \sum (\text{atomic mass} \times \text{subscript}) \).
Step 2: \( M(\text{Al}_2\text{S}_3) = 2 \times 27 + 3 \times 32 = 54 + 96 = 150 \text{ g/mol} \).
Step 3: Use the moles formula: \( n = \frac{m}{M} \).
Step 4: Substitute values: \( n = \frac{300.32}{150} = 2.002 \approx 2 \text{ mol} \).
Answer
Molar mass of Al₂S₃ = 150 g/mol. Number of moles in 300.32 g = 2 mol.
Example 3 (JEE/NEET Level) — Hydrolysis Reaction and Stoichiometry
Problem: Aluminium sulfide undergoes complete hydrolysis according to the reaction: \( \text{Al}_2\text{S}_3 + 6\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 + 3\text{H}_2\text{S} \). If 15.016 g of Al₂S₃ reacts completely with excess water, find: (a) the moles of H₂S produced and (b) the mass of Al(OH)₃ formed. (Molar mass: Al₂S₃ = 150.16 g/mol, Al(OH)₃ = 78 g/mol, H₂S = 34 g/mol)
Given: Mass of Al₂S₃ = 15.016 g, Molar mass of Al₂S₃ = 150.16 g/mol, Molar mass of Al(OH)₃ = 78 g/mol
Step 1: Calculate moles of Al₂S₃: \( n(\text{Al}_2\text{S}_3) = \frac{15.016}{150.16} = 0.1 \text{ mol} \).
Step 2: From the balanced equation, 1 mol Al₂S₃ produces 3 mol H₂S. So moles of H₂S = \( 0.1 \times 3 = 0.3 \text{ mol} \).
Step 3: Mass of H₂S = \( 0.3 \times 34 = 10.2 \text{ g} \).
Step 4: From the balanced equation, 1 mol Al₂S₃ produces 2 mol Al(OH)₃. So moles of Al(OH)₃ = \( 0.1 \times 2 = 0.2 \text{ mol} \).
Step 5: Mass of Al(OH)₃ = \( 0.2 \times 78 = 15.6 \text{ g} \).
Answer
(a) Moles of H₂S produced = 0.3 mol (mass = 10.2 g). (b) Mass of Al(OH)₃ formed = 15.6 g.
CBSE Exam Tips 2025-26 for Aluminium Sulfide Formula
- Memorise the criss-cross rule: We recommend practising the criss-cross method with at least 10 different ionic compounds. This rule directly applies to Al₂S₃ and is tested every year in CBSE Class 10 and 11 exams.
- Learn the hydrolysis equation: The reaction \( \text{Al}_2\text{S}_3 + 6\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 + 3\text{H}_2\text{S} \) is a favourite in CBSE Class 11 and JEE. Balance it correctly and note that H₂S gas is released.
- Molar mass calculation: In 2025-26 board exams, stoichiometry questions carry 3–5 marks. Always write the molar mass step clearly: \( M = 2(27) + 3(32) = 150 \text{ g/mol} \). Show all working for full marks.
- Physical properties to remember: Note the melting point (1100 °C), boiling point (~1500 °C), and density (2.02 g/cm³). These values appear in one-mark questions and fill-in-the-blank type questions.
- Solubility: Al₂S₃ is slightly soluble in water but completely soluble in acid. This distinction is tested in CBSE short-answer questions. Our experts suggest writing this as a key note in your revision sheet.
- IUPAC name: The IUPAC name is dialuminium trisulfide. Knowing this helps in Class 11 nomenclature questions and JEE multiple-choice questions.
Common Mistakes to Avoid with Aluminium Sulfide Formula
- Writing AlS instead of Al₂S₃: This is the most common error. Students forget to apply the criss-cross rule. Always use the ionic charges (+3 for Al, −2 for S) and cross-multiply to get Al₂S₃.
- Confusing sulfide (S²⁻) with sulfate (SO₄²⁻): Aluminium sulfide is Al₂S₃, while aluminium sulfate is Al₂(SO₄)₃. These are different compounds. Pay close attention to the anion name in the question.
- Incorrect molar mass calculation: Some students use the atomic mass of sulfur as 16 (the mass of oxygen) instead of 32. Remember: Sulfur (S) has an atomic mass of 32 g/mol.
- Unbalanced hydrolysis equation: Writing \( \text{Al}_2\text{S}_3 + \text{H}_2\text{O} \rightarrow \text{Al(OH)}_3 + \text{H}_2\text{S} \) without balancing is a common mistake. The correct balanced equation requires 6 water molecules, producing 2 Al(OH)₃ and 3 H₂S.
- Incorrect oxidation states: Students sometimes assign −1 to sulfur (as in sulfite) instead of −2 (as in sulfide). In Al₂S₃, sulfur is in the −2 oxidation state. This error leads to wrong formula derivation.
JEE/NEET Application of Aluminium Sulfide Formula
In our experience, JEE aspirants frequently encounter the Aluminium Sulfide Formula in questions on ionic compounds, stoichiometry, and hydrolysis reactions. NEET students meet it in the context of inorganic chemistry and Group 13 element reactions. Here are the three most common application patterns:
Pattern 1 — Stoichiometry and Mole Concept
JEE Main regularly tests mole concept calculations using Al₂S₃. A typical question asks for the mass of a product formed when a given mass of Al₂S₃ reacts with water or acid. The key formula is \( n = \frac{m}{M} \). Use the molar mass 150 g/mol and the balanced equation to find product moles.
Pattern 2 — Hydrolysis and Gas Evolution
Both JEE and NEET test the hydrolysis of Al₂S₃. The reaction \( \text{Al}_2\text{S}_3 + 6\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 + 3\text{H}_2\text{S} \uparrow \) produces hydrogen sulfide gas. Questions often ask which gas is evolved, or ask students to identify the type of reaction (double displacement / hydrolysis). Note that Al(OH)₃ is amphoteric — it can react with both acids and bases.
Pattern 3 — Oxidation State and Ionic Bonding
NEET questions on oxidation states frequently use Al₂S₃ as an example. Students must identify the oxidation state of aluminium (+3) and sulfur (−2) in the compound. JEE Advanced questions may ask about the structure of Al₂S₃ and compare it with other aluminium halides. Our experts suggest revising the electron configuration of Al (1s² 2s² 2p⁶ 3s² 3p¹) to understand why it forms a +3 ion. Aluminium loses its three valence electrons to achieve a stable noble gas configuration.
FAQs on Aluminium Sulfide Formula
We hope this comprehensive guide on the Aluminium Sulfide Formula has helped you understand the concept thoroughly. For more chemistry formulas, explore our Complete Chemistry Formulas Hub. You may also find these related articles useful: Ammonia Formula for understanding nitrogen compounds, Silicon Dioxide Formula for Group 14 compound comparisons, and Bleaching Powder Formula for more ionic compound practice. For the official NCERT syllabus and exam guidelines, visit ncert.nic.in.