Molar MassMolar mass of magnesium hydroxide (Mg(OH)2)
The molar mass of magnesium hydroxide, Mg(OH)2, is 58.32 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 58.32 grams.
58.32 g/mol
24.305 + 2 × 15.999 + 2 × 1.0080 = 58.319 · 5 atoms in the formula · also called milk of magnesia
About magnesium hydroxide
Magnesium hydroxide is a white solid that hardly dissolves in water. A suspension of it in water is sold as milk of magnesia, an antacid. One mole of it neutralises two moles of hydrochloric acid.
Other names: milk of magnesia.
The working
The formula Mg(OH)2 has 1 magnesium atom, 2 oxygen atoms and 2 hydrogen atoms, 5 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Magnesium (Mg) | 1 | 24.305 | 24.305 | 41.68% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 54.87% |
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 3.46% |
24.305 + 2 × 15.999 + 2 × 1.0080 = 58.319 g/mol
With the rounded atomic masses used in many school books (Mg = 24, O = 16, H = 1) the answer is 58 g/mol. The difference from 58.32 is small, but it shows why you should use the values your question paper gives.
Percentage composition
By mass, Mg(OH)2 is 41.68% magnesium, 54.87% oxygen and 3.46% hydrogen. The largest share comes from oxygen. In a 100 g sample there are 41.7 g of magnesium, 54.9 g of oxygen and 3.5 g of hydrogen.
Worked examples
Grams to moles. How many moles are in 40 g of Mg(OH)2? Divide the mass by the molar mass: 40 ÷ 58.32 = 0.6859 mol.
Moles to grams. What is the mass of 3 mol of Mg(OH)2? Multiply: 3 × 58.32 = 175 g.
Counting particles. 40 g contains 0.6859 mol × 6.022 × 1023 = 4.130 × 1023 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 5.832 g |
| 0.5 mol | 29.16 g |
| 1 mol | 58.32 g |
| 2 mol | 116.6 g |
| 5 mol | 291.6 g |
| Mass | Moles |
|---|---|
| 1 g | 0.01715 mol |
| 10 g | 0.1715 mol |
| 100 g | 1.715 mol |
| 1 kg | 17.15 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Mg(OH)2?
The molar mass of magnesium hydroxide (Mg(OH)2) is 58.32 g/mol, or 58.319 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Mg(OH)2?
100 ÷ 58.32 = 1.715 mol.
What is the mass of one formula unit of Mg(OH)2?
Divide the molar mass by the Avogadro constant: 58.32 ÷ 6.022 × 1023 = 9.684 × 10-23 g.
What is the percentage of oxygen in Mg(OH)2?
Oxygen makes up 54.87% of the mass of Mg(OH)2: 2 × 15.999 ÷ 58.319 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.