Molar MassMolar mass of barium hydroxide (Ba(OH)2)
The molar mass of barium hydroxide, Ba(OH)2, is 171.34 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 171.34 grams.
171.34 g/mol
137.33 + 2 × 15.999 + 2 × 1.0080 = 171.344 · 5 atoms in the formula · also called baryta
About barium hydroxide
Barium hydroxide is a strong base that dissolves fairly well in water. Its solution, baryta water, absorbs carbon dioxide to give insoluble barium carbonate. Barium is a heavy atom, so it supplies most of the mass.
Other names: baryta.
The working
The formula Ba(OH)2 has 1 barium 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 |
|---|---|---|---|---|
| Barium (Ba) | 1 | 137.33 | 137.330 | 80.15% |
| Oxygen (O) | 2 | 15.999 | 31.998 | 18.67% |
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 1.18% |
137.33 + 2 × 15.999 + 2 × 1.0080 = 171.344 g/mol
With the rounded atomic masses used in many school books (Ba = 137, O = 16, H = 1) the answer is 171 g/mol. The difference from 171.34 is too small to matter in most school sums.
Percentage composition
By mass, Ba(OH)2 is 80.15% barium, 18.67% oxygen and 1.18% hydrogen. The largest share comes from barium. In a 100 g sample there are 80.1 g of barium, 18.7 g of oxygen and 1.2 g of hydrogen.
Notice that hydrogen has more atoms in the formula (2) than barium (1), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.
Worked examples
Grams to moles. How many moles are in 25 g of Ba(OH)2? Divide the mass by the molar mass: 25 ÷ 171.34 = 0.1459 mol.
Moles to grams. What is the mass of 0.1 mol of Ba(OH)2? Multiply: 0.1 × 171.34 = 17.13 g.
Counting particles. 25 g contains 0.1459 mol × 6.022 × 1023 = 8.787 × 1022 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 171.34 = 17.13 g of Ba(OH)2 in each litre. For 250 mL, weigh 4.284 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 17.13 g |
| 0.5 mol | 85.67 g |
| 1 mol | 171.3 g |
| 2 mol | 342.7 g |
| 5 mol | 856.7 g |
| Mass | Moles |
|---|---|
| 1 g | 0.005836 mol |
| 10 g | 0.05836 mol |
| 100 g | 0.5836 mol |
| 1 kg | 5.836 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Ba(OH)2?
The molar mass of barium hydroxide (Ba(OH)2) is 171.34 g/mol, or 171.344 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Ba(OH)2?
100 ÷ 171.34 = 0.5836 mol.
What is the mass of one formula unit of Ba(OH)2?
Divide the molar mass by the Avogadro constant: 171.34 ÷ 6.022 × 1023 = 2.845 × 10-22 g.
What is the percentage of barium in Ba(OH)2?
Barium makes up 80.15% of the mass of Ba(OH)2: 1 × 137.33 ÷ 171.344 × 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.