Molar Mass of Magnesium Oxide (MgO)

Each row shows how much mass one element contributes to the total for MgO.

Save a printable summary, revision sheet, practice worksheet, or laboratory reference for Magnesium Oxide (MgO).

Without looking above, write the atom count for MgO, then compute the molar mass. Check your answer against 40.304 g/mol.

Next challenge: how many grams are in 0.250 mol of Magnesium Oxide? Multiply 0.250 × 40.304 to get 10.076 g.

Used in some flue gas desulfurization and wastewater treatment applications; naturally occurring in metamorphic mineral deposits without requiring synthesis for many uses.

Precursor to magnesium hydroxide antacid formulations; dietary magnesium oxide supplements provide a relatively concentrated, low-cost magnesium source, though with lower bioavailability than some other magnesium salts.

Preparation and production

Industrially produced by calcining (heating) magnesium carbonate or magnesium hydroxide at high temperature, driving off carbon dioxide or water respectively: MgCO₃ → MgO + CO₂, or Mg(OH)₂ → MgO + H₂O. Also obtained from seawater-derived magnesium hydroxide precipitated and subsequently calcined.

Global magnesium oxide production exceeds several million tonnes annually, sourced both from mined magnesite (magnesium carbonate) ore and from seawater/brine-derived magnesium hydroxide calcination, supplying refractory, agricultural, and construction material markets.

Magnesium oxide, in the form of magnesia, has been recognized and used since antiquity, with early references to magnesia alba (white magnesia, magnesium carbonate-derived material) in historical pharmacy and mineralogy. Joseph Black's mid-18th-century studies distinguishing magnesia from lime (calcium oxide) helped establish magnesium as a separate elemental system, and industrial-scale refractory-grade magnesia production expanded significantly with the growth of steelmaking in the late 19th and 20th centuries.

Recognized since antiquity as magnesia; Joseph Black's mid-18th-century chemical studies helped distinguish magnesium compounds from calcium compounds.

Interesting facts

• MgO's melting point of 2,852 °C is more than three times higher than that of NaCl, despite both sharing the same rock-salt crystal structure.
• Magnesia refractory bricks have lined steelmaking furnaces for well over a century, remaining essential to modern heavy industry.
• The mineral periclase, MgO's natural form, is a useful indicator mineral for geologists studying contact metamorphism in limestone deposits.
• Freshly burned magnesium metal produces a fine MgO smoke that, if inhaled in significant quantity, can cause a temporary flu-like illness known as metal fume fever — distinct from bulk MgO solid handling risk.

Comparison with similar compounds

MgO (40.30 g/mol, melting point 2,852 °C) and CaO (56.08 g/mol, melting point 2,613 °C) are both alkaline earth oxides sharing similar ionic bonding, but CaO reacts far more vigorously and exothermically with water than the comparatively sluggish MgO hydration.

Store in a dry, sealed container, as MgO slowly absorbs atmospheric moisture and carbon dioxide over long-term exposure, gradually converting surface material to magnesium hydroxide or carbonate. Stable under normal dry storage conditions otherwise.

Low acute hazard; treat fine powder as a nuisance dust and mild irritant. Use standard dust protection and eye protection when handling bulk refractory-grade material to avoid respiratory or eye irritation.

Generally low toxicity; used as a food additive, antacid precursor, and supplement at appropriate grades. Fine dust may cause mild respiratory or eye irritation.

• Mild respiratory irritation from inhaled fine powder ('metal fume fever' risk historically associated with freshly formed MgO fume from magnesium combustion, not bulk solid)
• Eye irritation from dust contact
• No significant acute toxicity at typical handling levels for bulk solid

Classification: Not classified as hazardous under GHS for standard bulk solid material

1. 1. Calculate the molar mass of MgO.

   Show answer

   24.30 + 16.00 = 40.30 g/mol

2. 2. How many grams of MgCl₂ form from the complete reaction of 20.15 g of MgO with excess HCl?

   Show answer

   20.15 g ÷ 40.30 g/mol = 0.500 mol MgO → 0.500 mol MgCl₂ × 95.21 g/mol ≈ 47.61 g

3. 3. What mass of CO₂ is released from calcining 84.31 g of MgCO₃ to form MgO?

   Hint: Molar mass of MgCO₃ = 24.30 + 12.01 + 3(16.00) = 84.31 g/mol.

   Show answer

   84.31 g ÷ 84.31 g/mol = 1.00 mol MgCO₃ → 1.00 mol CO₂ × 44.01 g/mol = 44.01 g

4. 4. Why does MgO have a much higher melting point than NaCl despite sharing the same crystal structure?

   Show answer

   MgO's ions carry double charges (Mg²⁺, O²⁻) rather than single charges, so the electrostatic lattice energy holding the crystal together is much greater, requiring far more energy to melt.

• Magnesium ChlorideMgCl₂

The sections above give the number you need for calculations. Here we look more closely at how Magnesium Oxide (MgO) behaves chemically—so the molar mass connects to real reactions, properties, and laboratory practice.

Magnesium oxide (MgO) is an ionic compound with molar mass 40.30 g/mol (Mg 24.30 + O 16.00), formed from Mg²⁺ and O²⁻ ions in a 1:1 ratio packed into the same rock-salt crystal structure as NaCl. Known industrially as magnesia, it is a white, odorless solid with an exceptionally high melting point (2,852 °C) driven by the strong electrostatic attraction between its doubly charged ions, a property that makes it one of the most important refractory materials used to line furnaces, kilns, and other equipment that must withstand extreme, sustained heat.

MgO's behavior in water reveals a useful chemical distinction from its more reactive alkaline earth relatives: it reacts only sluggishly with water at room temperature to form magnesium hydroxide, Mg(OH)₂ (MgO + H₂O → Mg(OH)₂), whereas calcium oxide reacts far more vigorously and exothermically with water. This relative sluggishness, combined with Mg(OH)₂'s own limited solubility, is precisely why "milk of magnesia" and related antacid formulations use magnesium hydroxide rather than magnesium oxide directly — the oxide must first slowly hydrate before it can act as the mild, buffered base that neutralizes stomach acid without the harsher, more rapid reactivity seen in some calcium-based antacids.

Beyond refractory and antacid applications, MgO serves as an essential agricultural soil amendment and animal feed mineral supplement (supplying dietary magnesium), an electrical insulator in mineral-insulated cables (exploiting its combination of high thermal conductivity and electrical resistance), and a component of specialty cements and construction boards. Its natural mineral form, periclase, occurs in metamorphosed limestones and provides a direct geological window into the same fundamental ionic bonding chemistry that makes synthetic magnesia industrially indispensable.

MgO pairs one Mg²⁺ ion with one O²⁻ ion in a 1:1 ratio, reflecting the straightforward charge balance between magnesium's +2 charge and oxygen's −2 charge. Like NaCl, it adopts a face-centered cubic rock-salt lattice, but the doubly charged ions create much stronger electrostatic (lattice) attraction, explaining MgO's dramatically higher melting point and hardness compared to singly charged ionic salts.

MgO is a basic oxide that reacts slowly with water to form magnesium hydroxide and more readily with acids to form magnesium salts and water (MgO + 2 HCl → MgCl₂ + H₂O). It is thermally extremely stable, remaining unreactive and solid at temperatures where most other common inorganic compounds have long since melted or decomposed — the basis of its refractory applications. It also reacts with atmospheric CO₂ only very slowly compared to more reactive alkaline earth oxides like CaO.

Recalculate any formula with the molar mass calculator, compare atoms on the periodic table, or browse more compounds in the oxide library.

• PubChem CID 14792: Magnesium oxide compound data
• NIST Chemistry WebBook: Thermodynamic properties
• USGS: Magnesium compounds and refractory material statistics