Molar MassMolar mass of aluminium hydroxide (Al(OH)3)
The molar mass of aluminium hydroxide, Al(OH)3, is 78.00 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 78.00 grams.
About aluminium hydroxide
Aluminium hydroxide is a white, jelly-like solid that is amphoteric: it reacts with both acids and strong alkalis. It is used in antacids and in purifying water. The bracket means three hydroxide groups for each aluminium.
The working
The formula Al(OH)3 has 1 aluminium atom, 3 oxygen atoms and 3 hydrogen atoms, 7 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Aluminium (Al) | 1 | 26.982 | 26.982 | 34.59% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 61.53% |
| Hydrogen (H) | 3 | 1.0080 | 3.024 | 3.88% |
26.982 + 3 × 15.999 + 3 × 1.0080 = 78.003 g/mol
With the rounded atomic masses used in many school books (Al = 27, O = 16, H = 1) the answer is 78 g/mol. The difference from 78.00 is too small to matter in most school sums.
Percentage composition
By mass, Al(OH)3 is 34.59% aluminium, 61.53% oxygen and 3.88% hydrogen. The largest share comes from oxygen. In a 100 g sample there are 34.6 g of aluminium, 61.5 g of oxygen and 3.9 g of hydrogen.
Worked examples
Grams to moles. How many moles are in 10 g of Al(OH)3? Divide the mass by the molar mass: 10 ÷ 78.00 = 0.1282 mol.
Moles to grams. What is the mass of 1.5 mol of Al(OH)3? Multiply: 1.5 × 78.00 = 117 g.
Counting particles. 10 g contains 0.1282 mol × 6.022 × 1023 = 7.720 × 1022 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 7.8 g |
| 0.5 mol | 39 g |
| 1 mol | 78 g |
| 2 mol | 156 g |
| 5 mol | 390 g |
| Mass | Moles |
|---|---|
| 1 g | 0.01282 mol |
| 10 g | 0.1282 mol |
| 100 g | 1.282 mol |
| 1 kg | 12.82 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Al(OH)3?
The molar mass of aluminium hydroxide (Al(OH)3) is 78.00 g/mol, or 78.003 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Al(OH)3?
100 ÷ 78.00 = 1.282 mol.
What is the mass of one formula unit of Al(OH)3?
Divide the molar mass by the Avogadro constant: 78.00 ÷ 6.022 × 1023 = 1.295 × 10-22 g.
What is the percentage of oxygen in Al(OH)3?
Oxygen makes up 61.53% of the mass of Al(OH)3: 3 × 15.999 ÷ 78.003 × 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.