Molar Mass of Magnesium Chloride (MgCl₂)
Learn how chemists calculate the molar mass of Magnesium Chloride (MgCl₂), with a clear formula breakdown, worked steps, and study notes · IUPAC name: Magnesium chloride.
Quick answer
The molar mass of Magnesium Chloride (MgCl₂) is
95.205g/mol
One mole of Magnesium Chloride therefore has a mass of 95.205 grams—the value you use for stoichiometry and laboratory preparation.
Reviewed for educational accuracy · Accuracy policy
- CAS Registry Number
- 7786-30-3
- PubChem CID
- 5360315
- SMILES
- [Mg+2].[Cl-].[Cl-]
Step-by-step calculation
Let's find the molar mass of Magnesium Chloride (MgCl₂) together—step by step, as if you are seeing the formula for the first time.
Step 1 — Look at the chemical formula
The formula is MgCl₂. Each letter stands for an element. The little number after a letter (the subscript) tells you how many atoms of that element are in one molecule or formula unit.
- 1 Magnesium atom (Mg)
- 2 Chlorine atoms (Cl)
Step 2 — Look up each atomic mass
Atomic mass comes from the periodic table. It is the average mass of one mole of atoms of that element, in grams per mole (g/mol). Think of it as the "price tag" for one mole of that element.
- Magnesium (Mg) = 24.305 g/mol
- Chlorine (Cl) = 35.450 g/mol
Step 3 — Multiply atoms × atomic mass
Why multiply? If one oxygen atom "costs" about 16 g/mol, then two oxygen atoms cost twice as much. Each element's contribution is: number of atoms × atomic mass.
- 1 × 24.305 = 24.305 g/mol (Magnesium)
- 2 × 35.450 = 70.900 g/mol (Chlorine)
Step 4 — Add the contributions
Why add? The molar mass of the whole compound is simply the total mass of every atom in the formula. Add each element's contribution:
24.305 + 70.900 = 95.205 g/mol
Step 5 — Final answer
Molar mass of Magnesium Chloride = 95.205 g/mol
That means one mole of Magnesium Chloride (MgCl₂) has a mass of about 95.21 grams.
Quick summary
Read the formula → count atoms → look up atomic masses → multiply → add → report g/mol. For MgCl₂, the total is 95.205 g/mol.
Common beginner mistakes
- Using van't Hoff factor i = 2 for MgCl₂ colligative property calculations — it should be i ≈ 3 (one Mg²⁺ plus two Cl⁻).
- Confusing MgCl₂ molar mass with MgSO₄ (Epsom salt, 120.37 g/mol anhydrous) — different anions entirely.
- Forgetting the hexahydrate's added water mass when working with commercial deicing or nigari-grade material.
Memory trick
Charge-balance Mg²⁺ with two Cl⁻ to derive the formula rather than memorizing it.
Mini practice
Without looking above, list the atoms in MgCl₂ and write one multiplication line for the heaviest element. Then check your work against Step 3.
Real-world example
If a recipe asks for 0.100 mol of Magnesium Chloride, mass needed = 0.100 × 95.205 = 9.521 g. That is how chemists turn a mole amount into a weighable sample.
Atomic contribution table
Each row shows how much mass one element contributes to the total for MgCl₂.
| Element | Atoms | Atomic mass | Contribution | Mass % |
|---|---|---|---|---|
| Mg | 1 | 24.305 | 24.305 g/mol | 25.5% |
| Cl | 2 | 35.450 | 70.900 g/mol | 74.5% |
| Total molar mass | 95.205 g/mol | 100% | ||
Mass contribution chart
Teal and blue circles alternate like positive and negative ions in a crystal lattice. Formula mass is the mass of one formula unit, not a single molecule.
Download study sheets
Save a printable summary, revision sheet, practice worksheet, or laboratory reference for Magnesium Chloride (MgCl₂).
Practice this calculation
Without looking above, write the atom count for MgCl₂, then compute the molar mass. Check your answer against 95.205 g/mol.
Next challenge: how many grams are in 0.250 mol of Magnesium Chloride? Multiply 0.250 × 95.205 to get 23.801 g.
Physical and chemical properties
Physical properties
| Appearance | White to colorless crystalline solid (anhydrous); white deliquescent crystals (hexahydrate) |
| Color | White to colorless |
| Odor | Odorless |
| State (STP) | Solid |
| Density | 2.32 g/cm³ (anhydrous); 1.57 g/cm³ (hexahydrate) |
| Melting point | 714 °C (anhydrous); 118 °C (hexahydrate, loses water) |
| Boiling point | 1,412 °C (anhydrous, decomposes) |
| Solubility | 543 g/L water at 20 °C (highly soluble, deliquescent) |
| Crystal structure | Trigonal/rhombohedral (anhydrous, CdCl₂-type layered structure) |
Chemical properties
| Classification | Ionic salt / alkaline earth halide |
| Family | Group 2 halide (alkaline earth chloride) |
| Polarity | Ionic |
| Oxidation states | Mg: +2, Cl: −1 |
Applications
Industrial uses
- Road and runway deicing brine (effective at low temperatures)
- Dust suppression and soil stabilization on unpaved roads
- Magnesium metal production (historically via molten salt electrolysis)
- Wastewater treatment coagulant and fire-retardant textile treatment
Laboratory uses
- Soluble magnesium ion source for synthesis and buffer preparation
- Demonstrating deliquescence and hydrate chemistry
- Electrolysis experiments (molten salt decomposition)
Deicing brine runoff elevates chloride and magnesium in soil and waterways; naturally abundant in seawater and some inland brine deposits without requiring synthesis.
Nigari-derived MgCl₂ coagulates tofu; dietary magnesium supplement; essential cofactor for numerous enzymatic processes in living organisms.
Preparation and production
Recovered from seawater or brine evaporation (bittern) or mined from marine evaporite deposits such as bischofite. Can be prepared in the laboratory by reacting magnesium carbonate, oxide, or hydroxide with hydrochloric acid, followed by crystallization as the hexahydrate.
Global magnesium chloride production is closely tied to solar salt and potash operations that process seawater or inland brines; the Great Salt Lake, the Dead Sea, and various solar evaporation ponds are major commercial sources.
Important reactions of Magnesium Chloride
MgCl₂(l) → Mg(l) + Cl₂(g)
- Reaction type
- Electrolysis (molten)
- Conditions
- Molten MgCl₂, industrial electrolytic cell
- Explanation
- Electrical energy decomposes molten magnesium chloride into magnesium metal at the cathode and chlorine gas at the anode — the classic industrial route to magnesium metal.
- Products
- Magnesium metal and chlorine gas
- Why it matters
- Industrial magnesium metal production
Related ideas: Electrolysis · Redox reactions · Metal extraction
MgCl₂(aq) + 2 NaOH(aq) → Mg(OH)₂(s) + 2 NaCl(aq)
- Reaction type
- Precipitation (double displacement)
- Conditions
- Aqueous, room temperature
- Explanation
- Hydroxide ion precipitates insoluble magnesium hydroxide, a reaction used both analytically and in extracting magnesium from seawater via the Dow process.
- Products
- Magnesium hydroxide precipitate and sodium chloride
- Why it matters
- Magnesium extraction from seawater, water treatment
Related ideas: Solubility rules · Precipitation reactions · Industrial extraction
MgCO₃(s) + 2 HCl(aq) → MgCl₂(aq) + H₂O(l) + CO₂(g)
- Reaction type
- Acid–carbonate
- Conditions
- Aqueous, room temperature
- Explanation
- Hydrochloric acid dissolves magnesium carbonate to produce magnesium chloride solution with carbon dioxide effervescence, a laboratory preparation route.
- Products
- Magnesium chloride, water, and carbon dioxide
- Why it matters
- Laboratory MgCl₂ preparation
Related ideas: Acid–carbonate reactions · Gas evolution · Salt preparation
Soy protein (colloid) + Mg²⁺ → Coagulated protein network (tofu curd)
- Reaction type
- Protein coagulation (colloid chemistry)
- Conditions
- Hot soy milk, controlled addition of MgCl₂ (nigari)
- Explanation
- Divalent magnesium ions cross-link negatively charged soy protein particles, destabilizing the colloidal suspension and causing it to aggregate into a curd that traps water — the chemistry behind traditional tofu-making.
- Products
- Coagulated tofu curd and whey
- Why it matters
- Traditional tofu production
Related ideas: Colloid chemistry · Protein coagulation · Food chemistry
History and discovery
Bittern and bischofite have been recognized as sources of magnesium salts since early salt-production and tofu-making traditions in East Asia. The systematic industrial extraction of magnesium metal from seawater-derived MgCl₂ was pioneered in the early 20th century, with the Dow Chemical Company establishing large-scale seawater magnesium production in the 1940s that supplied magnesium for lightweight aircraft alloys during World War II.
Magnesium salts have been used since antiquity in food processing (nigari); systematic industrial magnesium chloride electrolysis for metal production developed in the early-to-mid 20th century.
Interesting facts
- MgCl₂ is the second most abundant salt in seawater by mass, after sodium chloride.
- Nigari, the traditional Japanese tofu coagulant, is essentially concentrated magnesium chloride bittern left over from sea salt production.
- Some cities pre-treat roads with magnesium chloride brine before snowstorms because it lowers the freezing point more effectively at very cold temperatures than rock salt.
- The historic Dow process could, in principle, extract magnesium metal from an essentially inexhaustible resource: ordinary seawater.
Comparison with similar compounds
MgCl₂ (95.21 g/mol) and CaCl₂ (110.98 g/mol) are both widely used deicing salts; MgCl₂ is generally effective at somewhat lower temperatures, while CaCl₂ releases more heat on dissolution and is often preferred for very cold conditions.
Storage, handling, and safety
Store the hexahydrate in sealed, moisture-resistant containers to prevent further water absorption or caking. Anhydrous MgCl₂ must be kept rigorously dry, ideally under inert atmosphere, to avoid hydrolysis to magnesium oxychloride.
Low acute hazard; treat concentrated solutions and dust as mild irritants. Standard gloves and eye protection are adequate for typical laboratory or industrial handling.
Low toxicity; generally recognized as safe as a food additive and supplement at recommended doses. Concentrated brine may irritate skin and eyes.
- Mild eye and skin irritation from concentrated solutions or dust
- Excessive magnesium intake can cause gastrointestinal upset
- Corrosive to some metals and concrete formulations at high brine concentrations over time
Classification: Not classified as hazardous under GHS for standard-grade material
Exam notes and student tips
Exam notes
- Molar mass MgCl₂ = 24.31 + 2(35.45) = 95.21 g/mol.
- Molar mass hexahydrate MgCl₂·6H₂O = 95.21 + 6(18.02) = 203.33 g/mol.
- MgCl₂ is a strong electrolyte: 1 mol dissolved → 1 mol Mg²⁺ + 2 mol Cl⁻ (van't Hoff factor i ≈ 3).
- Electrolysis of molten MgCl₂: cathode Mg metal, anode Cl₂ gas.
Student tips
- Charge-balance Mg²⁺ with two Cl⁻ to derive the formula rather than memorizing it.
- Link MgCl₂'s deliquescence to its strong ion-dipole attraction for water molecules.
- Remember nigari (tofu coagulant) as a memorable real-world example of divalent cation protein coagulation.
Common mistakes
- Using van't Hoff factor i = 2 for MgCl₂ colligative property calculations — it should be i ≈ 3 (one Mg²⁺ plus two Cl⁻).
- Confusing MgCl₂ molar mass with MgSO₄ (Epsom salt, 120.37 g/mol anhydrous) — different anions entirely.
- Forgetting the hexahydrate's added water mass when working with commercial deicing or nigari-grade material.
Misconceptions
- MgCl₂ and MgSO₄ (Epsom salt) are not interchangeable — they have different anions, solubilities, and typical uses.
- Nigari is not a chemical additive foreign to tofu tradition — it is simply concentrated magnesium chloride from seawater bittern, used for centuries.
- Anhydrous MgCl₂ is not simply made by heating the hydrate in open air — this can partially hydrolyze it to magnesium oxychloride instead of pure anhydrous salt.
Practice questions
1. Calculate the molar mass of MgCl₂.
Show answer
24.31 + 2(35.45) = 95.21 g/mol
2. What is the molar mass of the hexahydrate MgCl₂·6H₂O?
Show answer
95.21 + 6(18.02) = 203.33 g/mol
3. How many moles of Cl⁻ are released when 19.04 g of MgCl₂ fully dissociates in water?
Show answer
19.04 g ÷ 95.21 g/mol = 0.200 mol MgCl₂ → 0.200 × 2 = 0.400 mol Cl⁻
4. What is the van't Hoff factor (i) for MgCl₂ in dilute solution?
Show answer
i ≈ 3, because each formula unit dissociates into one Mg²⁺ and two Cl⁻ ions.
Frequently asked questions about Magnesium Chloride
95.21 g/mol for anhydrous MgCl₂; 203.33 g/mol for the common hexahydrate MgCl₂·6H₂O.
Chemistry of Magnesium Chloride
The sections above give the number you need for calculations. Here we look more closely at how Magnesium Chloride (MgCl₂) behaves chemically—so the molar mass connects to real reactions, properties, and laboratory practice.
Magnesium chloride (MgCl₂) is an ionic compound with molar mass 95.21 g/mol (Mg 24.31 + Cl 2 × 35.45), formed from one Mg²⁺ ion balancing two Cl⁻ ions. It is intensely hygroscopic, and the common hexahydrate form MgCl₂·6H₂O (bischofite) picks up atmospheric moisture so readily that it is prized as a deicing agent — it lowers the freezing point of water and, unlike sodium chloride, remains effective at much lower temperatures while being comparatively gentler on some types of concrete and vegetation.
MgCl₂ is a major solute in seawater (after NaCl, it is the second most abundant salt by mass) and is recovered on a huge industrial scale by evaporating seawater or brine from inland deposits like Utah's Great Salt Lake. This same abundance makes bittern — the magnesium-chloride-rich mother liquor left over after sea salt crystallizes out of seawater — the traditional coagulant used in Japanese and Chinese tofu-making (nigari), where Mg²⁺ ions cross-link soy protein molecules to curdle soy milk into a firm gel, a food-chemistry application with roots stretching back over a thousand years.
Beyond deicing and tofu production, MgCl₂ is a convenient, highly soluble source of magnesium ion for chemical synthesis, magnesium metal production (historically via the Dow process, electrolyzing molten MgCl₂ recovered from seawater), dust control on unpaved roads, and dietary magnesium supplementation, since Mg²⁺ is an essential cofactor for hundreds of enzymes involved in energy metabolism, DNA replication, and neuromuscular function.
MgCl₂ pairs one Mg²⁺ ion (losing two electrons from its outer shell) with two Cl⁻ ions (each gaining one electron) to achieve overall charge neutrality. The formula's subscript 2 on chlorine directly reflects the +2 charge of magnesium — a standard example of using ionic charge balance to derive a formula. In its common hexahydrate form, six water molecules coordinate around each formula unit.
MgCl₂ is a neutral, highly soluble salt that dissociates completely in water. It is deliquescent, readily absorbing atmospheric moisture to form saturated solutions. Molten MgCl₂ undergoes electrolysis to yield magnesium metal and chlorine gas (historically the Dow process route to magnesium). It reacts with strong bases to precipitate insoluble magnesium hydroxide, and with soy proteins as a coagulant through calcium/magnesium cross-linking of protein networks, similar in principle to how calcium sulfate coagulates tofu in other traditional recipes.
Deicing chemistry and cold-weather advantage over NaCl
Magnesium chloride brine is widely used for road deicing because it remains effective at significantly lower temperatures than sodium chloride and produces less chloride-driven corrosion of some infrastructure per unit of deicing power, though it still contributes to environmental chloride loading. Its strong hygroscopicity also helps it pre-wet road surfaces before storms, improving deicing performance.
Nigari and traditional tofu coagulation
Bittern, the magnesium-chloride-rich residual brine left after crystallizing sea salt from seawater, is the source of nigari, the traditional Japanese coagulant added to hot soy milk to curdle it into tofu. Magnesium ions cross-link negatively charged soy protein molecules, causing them to aggregate into a gel network that traps water — the same fundamental chemistry (divalent cation-induced protein coagulation) used with calcium sulfate in some tofu-making traditions.
Seawater abundance and the Dow magnesium process
Magnesium chloride is the second most abundant dissolved salt in seawater by mass after sodium chloride. The historic Dow process extracted magnesium metal directly from seawater by precipitating magnesium hydroxide, converting it to magnesium chloride, and electrolyzing the molten salt — a striking example of turning an essentially unlimited marine resource into structural metal.
Dietary magnesium source and physiological role
As a highly soluble, well-tolerated magnesium salt, MgCl₂ is used in dietary supplements and oral rehydration-style formulations to correct magnesium deficiency. Magnesium ion itself is an essential cofactor for over 300 enzymes, including those in ATP metabolism, DNA and RNA synthesis, and neuromuscular signal transmission, making adequate magnesium intake important for numerous physiological processes.
Hexahydrate vs. anhydrous forms in practice
Commercial magnesium chloride is most often sold and used as the hexahydrate, MgCl₂·6H₂O, because the anhydrous salt is so strongly hygroscopic that it is difficult to produce and store without picking up water; heating the hydrate to drive off water can also partially hydrolyze it to magnesium oxychloride, so anhydrous MgCl₂ for magnesium metal production is typically made by more controlled dehydration or direct chlorination routes.
Recalculate any formula with the molar mass calculator, compare atoms on the periodic table, or browse more compounds in the salt library.
References and further reading
- PubChem CID 5360315: Magnesium chloride compound data
- NIST Chemistry WebBook: Thermodynamic properties
- USGS: Magnesium compounds and brine resource statistics