The Magnesium Chloride Formula, written as MgCl2, represents one of the most important ionic compounds studied in NCERT Chemistry for Classes 10 and 11. This compound forms when magnesium reacts with chlorine, and it appears across CBSE board exams, JEE Main, and NEET in the context of ionic bonding, electrolysis, and chemical reactions. In this article, we cover the formula derivation, molecular structure, physical and chemical properties, solved examples at three difficulty levels, CBSE exam tips for 2025-26, and JEE/NEET application patterns.
Key Magnesium Chloride Formulas at a Glance
Quick reference for the most important formulas and facts about MgCl₂.
- Molecular formula: \( \text{MgCl}_2 \)
- Molar mass: \( M = 24.31 + 2 \times 35.45 = 95.21 \text{ g/mol} \)
- Formation reaction: \( \text{Mg} + \text{Cl}_2 \rightarrow \text{MgCl}_2 \)
- Ionic charges: \( \text{Mg}^{2+} \) and \( \text{Cl}^{-} \)
- Percent composition of Mg: \( \%\text{Mg} = \frac{24.31}{95.21} \times 100 \approx 25.54\% \)
- Hydrated form: \( \text{MgCl}_2 \cdot 6\text{H}_2\text{O} \) (hexahydrate)
- Electrolysis: \( \text{MgCl}_2 \xrightarrow{\text{electrolysis}} \text{Mg} + \text{Cl}_2 \)
What is the Magnesium Chloride Formula?
The Magnesium Chloride Formula is \( \text{MgCl}_2 \). It is the chemical formula for an ionic compound formed by the combination of magnesium (Mg) and chlorine (Cl). Magnesium belongs to Group 2 of the periodic table and has an atomic number of 12. It readily loses two electrons to form the \( \text{Mg}^{2+} \) cation. Chlorine belongs to Group 17 and gains one electron to form the \( \text{Cl}^{-} \) anion.
To balance the charges, two chloride ions are needed for every one magnesium ion. This gives the formula \( \text{MgCl}_2 \). The compound is covered in NCERT Chemistry Class 10 (Chapter 3: Metals and Non-metals) and revisited in Class 11 (Chapter 4: Chemical Bonding and Molecular Structure). It is also referenced in Class 11 Chapter 10 (The s-Block Elements) in the context of alkaline earth metals.
Magnesium chloride is a white, crystalline solid at room temperature. It is highly soluble in water and dissociates completely into ions in aqueous solution. Its wide industrial applications — from road de-icing to tofu production — make it a frequently tested compound in both board and competitive exams.
Magnesium Chloride Formula — Expression and Variables
The molecular formula for magnesium chloride is:
\[ \text{MgCl}_2 \]
The molar mass is calculated as:
\[ M(\text{MgCl}_2) = M(\text{Mg}) + 2 \times M(\text{Cl}) = 24.31 + 2 \times 35.45 = 95.21 \text{ g/mol} \]
| Symbol / Component | Identity | Charge / Value | SI Unit |
|---|---|---|---|
| Mg | Magnesium ion (cation) | +2 | — |
| Cl | Chloride ion (anion) | −1 each | — |
| M(MgCl₂) | Molar mass of compound | 95.21 | g/mol |
| Melting point | Temperature of fusion | 714 | °C |
| Boiling point | Temperature of vaporisation | 1412 | °C |
| Density | Mass per unit volume | 2.32 | g/cm³ |
| Solubility in water | At 20°C | 54.3 | g/100 mL |
Derivation of the Magnesium Chloride Formula
Magnesium has the electronic configuration 2, 8, 2. It loses its two valence electrons to achieve a stable noble-gas configuration. This forms the \( \text{Mg}^{2+} \) ion with a charge of +2.
Chlorine has the electronic configuration 2, 8, 7. It needs one electron to complete its octet. So it gains one electron to form the \( \text{Cl}^{-} \) ion with a charge of −1.
To balance the charges, two \( \text{Cl}^{-} \) ions are needed for one \( \text{Mg}^{2+} \) ion. The total positive charge is +2 and the total negative charge is \( 2 \times (-1) = -2 \). The charges cancel, giving a neutral compound. Therefore, the formula is \( \text{MgCl}_2 \).
The formation reaction is:
\[ \text{Mg} + \text{Cl}_2 \rightarrow \text{MgCl}_2 \]
Complete Chemistry Formula Sheet — Ionic Compounds and Related Formulas
| Formula Name | Expression | Variables / Notes | Molar Mass (g/mol) | NCERT Chapter |
|---|---|---|---|---|
| Magnesium Chloride | \( \text{MgCl}_2 \) | Mg²⁺ + 2Cl⁻; ionic compound | 95.21 | Class 10, Ch 3; Class 11, Ch 10 |
| Magnesium Oxide | \( \text{MgO} \) | Mg²⁺ + O²⁻; basic oxide | 40.30 | Class 10, Ch 3 |
| Magnesium Hydroxide | \( \text{Mg(OH)}_2 \) | Mg²⁺ + 2OH⁻; milk of magnesia | 58.32 | Class 10, Ch 2 |
| Magnesium Sulphate | \( \text{MgSO}_4 \) | Mg²⁺ + SO₄²⁻; Epsom salt | 120.37 | Class 11, Ch 10 |
| Barium Acetate | \( \text{Ba(CH}_3\text{COO)}_2 \) | Ba²⁺ + 2CH₃COO⁻ | 255.43 | Class 11, Ch 10 |
| Zinc Bromide | \( \text{ZnBr}_2 \) | Zn²⁺ + 2Br⁻; similar to MgCl₂ | 225.18 | Class 11, Ch 12 |
| Ammonium Acetate | \( \text{NH}_4\text{CH}_3\text{COO} \) | NH₄⁺ + CH₃COO⁻; buffer salt | 77.08 | Class 11, Ch 7 |
| Calcium Chloride | \( \text{CaCl}_2 \) | Ca²⁺ + 2Cl⁻; similar structure to MgCl₂ | 110.98 | Class 10, Ch 2 |
| Sodium Chloride | \( \text{NaCl} \) | Na⁺ + Cl⁻; common salt | 58.44 | Class 10, Ch 2 |
| Aluminium Chloride | \( \text{AlCl}_3 \) | Al³⁺ + 3Cl⁻; covalent character | 133.34 | Class 11, Ch 11 |
| Molar Mass Formula | \( M = \sum n_i \times A_i \) | nᵢ = number of atoms, Aᵢ = atomic mass | — | Class 11, Ch 1 |
| Percent Composition | \( \% = \frac{\text{mass of element}}{M} \times 100 \) | Used for empirical formula problems | — | Class 11, Ch 1 |
Magnesium Chloride Formula — Solved Examples
Example 1 (Class 9-10 Level) — Calculating Molar Mass
Problem: Calculate the molar mass of magnesium chloride (MgCl₂). Given: Atomic mass of Mg = 24.31 g/mol, Atomic mass of Cl = 35.45 g/mol.
Given: Mg = 24.31 g/mol, Cl = 35.45 g/mol, Formula = MgCl₂
Step 1: Write the formula: \( \text{MgCl}_2 \) contains 1 Mg atom and 2 Cl atoms.
Step 2: Apply the molar mass formula: \( M = M(\text{Mg}) + 2 \times M(\text{Cl}) \)
Step 3: Substitute values: \( M = 24.31 + 2 \times 35.45 \)
Step 4: Calculate: \( M = 24.31 + 70.90 = 95.21 \text{ g/mol} \)
Answer
The molar mass of MgCl₂ = 95.21 g/mol
Example 2 (Class 11-12 Level) — Percentage Composition and Moles
Problem: Find the percentage composition of magnesium in MgCl₂. Also, calculate the number of moles in 47.605 g of MgCl₂.
Given: Mass of MgCl₂ = 47.605 g, Molar mass of MgCl₂ = 95.21 g/mol, M(Mg) = 24.31 g/mol
Step 1: Use the percentage composition formula: \( \%\text{Mg} = \frac{M(\text{Mg})}{M(\text{MgCl}_2)} \times 100 \)
Step 2: Substitute: \( \%\text{Mg} = \frac{24.31}{95.21} \times 100 \approx 25.54\% \)
Step 3: Calculate percentage of Cl: \( \%\text{Cl} = 100 – 25.54 = 74.46\% \)
Step 4: Calculate moles: \( n = \frac{\text{mass}}{\text{molar mass}} = \frac{47.605}{95.21} = 0.5 \text{ mol} \)
Step 5: Number of formula units: \( N = n \times N_A = 0.5 \times 6.022 \times 10^{23} = 3.011 \times 10^{23} \)
Answer
Percentage of Mg = 25.54%, Percentage of Cl = 74.46%, Number of moles = 0.5 mol, Formula units = 3.011 × 10²³
Example 3 (JEE/NEET Level) — Electrolysis and Stoichiometry
Problem: Molten MgCl₂ is electrolysed using a current of 5 A for 1930 seconds. Calculate the mass of magnesium deposited at the cathode. (Faraday’s constant F = 96500 C/mol, M(Mg) = 24.31 g/mol)
Given: Current I = 5 A, Time t = 1930 s, n-factor of Mg²⁺ = 2, F = 96500 C/mol
Step 1: Write the cathode half-reaction: \( \text{Mg}^{2+} + 2e^{-} \rightarrow \text{Mg} \)
Step 2: Calculate total charge passed: \( Q = I \times t = 5 \times 1930 = 9650 \text{ C} \)
Step 3: Calculate moles of electrons: \( n_{e^-} = \frac{Q}{F} = \frac{9650}{96500} = 0.1 \text{ mol} \)
Step 4: Since 2 moles of electrons deposit 1 mole of Mg, moles of Mg deposited: \( n_{\text{Mg}} = \frac{0.1}{2} = 0.05 \text{ mol} \)
Step 5: Mass of Mg deposited: \( m = n \times M = 0.05 \times 24.31 = 1.2155 \approx 1.22 \text{ g} \)
Answer
Mass of magnesium deposited = 1.22 g
CBSE Exam Tips 2025-26 for Magnesium Chloride Formula
- Always write the correct subscript: The formula is MgCl2 — not MgCl or Mg2Cl. A wrong subscript loses full marks in CBSE answer scripts.
- Show the ionic charge balance: In questions asking you to derive the formula, always show Mg²⁺ and 2Cl⁻ separately. This demonstrates understanding and earns step marks.
- Molar mass calculation: We recommend learning the atomic masses of Mg (24.31) and Cl (35.45) by heart. These appear in stoichiometry and mole concept questions every year.
- Distinguish anhydrous from hydrated forms: CBSE 2025-26 papers may ask about MgCl₂·6H₂O (hexahydrate). Know that its molar mass = 95.21 + 6 × 18 = 203.21 g/mol.
- Reaction with water: MgCl₂ dissolves in water to give a slightly acidic solution due to hydrolysis of Mg²⁺. This concept appears in the ionic equilibrium chapter.
- Industrial uses earn marks: CBSE frequently asks 1-mark questions on uses. Remember: road de-icing, dust control, tofu coagulant, fertiliser, and fireproofing agents.
Common Mistakes to Avoid with the Magnesium Chloride Formula
- Mistake 1 — Writing MgCl instead of MgCl₂: Students sometimes forget that Mg has a +2 charge and needs two Cl⁻ ions. Always check that charges balance before writing the final formula.
- Mistake 2 — Confusing Mg²⁺ with Na⁺: Sodium chloride is NaCl (1:1 ratio) because Na has a +1 charge. Magnesium chloride is MgCl₂ (1:2 ratio) because Mg has a +2 charge. Do not mix these up.
- Mistake 3 — Incorrect molar mass: A common error is using 24 instead of 24.31 for Mg, or 35 instead of 35.45 for Cl. Use exact atomic masses in calculations to avoid rounding errors.
- Mistake 4 — Forgetting the n-factor in electrolysis: In electrolysis problems, students often use n-factor = 1 for Mg²⁺. The correct n-factor is 2, since Mg²⁺ gains 2 electrons at the cathode.
- Mistake 5 — Confusing MgCl₂ with MgO or Mg(OH)₂: All three are magnesium compounds, but they have different formulas and properties. Read the question carefully before writing the formula.
JEE/NEET Application of the Magnesium Chloride Formula
In our experience, JEE aspirants encounter the Magnesium Chloride Formula in at least three major topic areas. Understanding these patterns helps you score efficiently.
Pattern 1 — Mole Concept and Stoichiometry (JEE Main, NEET)
Questions often give a certain mass of MgCl₂ and ask for the number of moles, number of ions, or mass of a product formed. The key formula is:
\[ n = \frac{m}{M} = \frac{m}{95.21} \]
In aqueous solution, 1 mole of MgCl₂ gives 1 mole of Mg²⁺ and 2 moles of Cl⁻. So the total moles of ions = 3n. This fact is tested in colligative property questions in JEE Main.
Pattern 2 — Electrolysis (JEE Main, Class 12 Electrochemistry)
The electrolysis of molten MgCl₂ is the industrial method for producing magnesium metal (Dow process). JEE problems test Faraday’s laws here. The half-reactions are:
\[ \text{Cathode: } \text{Mg}^{2+} + 2e^{-} \rightarrow \text{Mg} \]
\[ \text{Anode: } 2\text{Cl}^{-} \rightarrow \text{Cl}_2 + 2e^{-} \]
Always remember: the n-factor for Mg²⁺ is 2. This is a high-frequency error point in JEE Main.
Pattern 3 — Ionic Bonding and Lattice Energy (JEE Advanced)
JEE Advanced tests the comparison of lattice energies across ionic compounds. MgCl₂ has a higher lattice energy than NaCl because the Mg²⁺ ion has a higher charge density. The Born-Haber cycle for MgCl₂ is a classic JEE Advanced problem. Our experts suggest practising the full Born-Haber cycle for MgCl₂ at least twice before the exam.
For NEET, the Magnesium Chloride Formula appears in the context of the biological role of Mg²⁺ ions — particularly in chlorophyll (which contains Mg²⁺ at its centre) and in enzyme activation. NEET questions may ask about the coordination of Mg²⁺ in biological systems.
FAQs on Magnesium Chloride Formula
For more chemistry formula resources, explore our complete Chemistry Formulas hub on ncertbooks.net. You may also find these related articles useful: the Barium Acetate Formula for understanding polyatomic ionic compounds, the Zinc Bromide Formula for a comparison with similar dihalide structures, and the Ammonium Acetate Formula for buffer salt chemistry. For the official NCERT syllabus reference, visit ncert.nic.in.