Molar MassMolar mass of sodium hydroxide (NaOH)
The molar mass of sodium hydroxide, NaOH, is 40.00 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 40.00 grams.
40.00 g/mol
22.990 + 15.999 + 1.0080 = 39.997 · 3 atoms in the formula · also called caustic soda
About sodium hydroxide
Sodium hydroxide is a white solid that dissolves in water with a lot of heat to give a strong alkali. It is used to make soap, paper and many chemicals. The pellets take up water and carbon dioxide from the air, so it cannot be weighed out as a primary standard.
Other names: caustic soda.
The working
The formula NaOH has 1 sodium atom, 1 oxygen atom and 1 hydrogen atom, 3 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Sodium (Na) | 1 | 22.990 | 22.990 | 57.48% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 40.00% |
| Hydrogen (H) | 1 | 1.0080 | 1.008 | 2.52% |
22.990 + 15.999 + 1.0080 = 39.997 g/mol
With the rounded atomic masses used in many school books (Na = 23, O = 16, H = 1) the answer is 40 g/mol. The difference from 40.00 is too small to matter in most school sums.
Percentage composition
By mass, NaOH is 57.48% sodium, 40.00% oxygen and 2.52% hydrogen. The largest share comes from sodium. In a 100 g sample there are 57.5 g of sodium, 40.0 g of oxygen and 2.5 g of hydrogen.
Worked examples
Grams to moles. How many moles are in 100 g of NaOH? Divide the mass by the molar mass: 100 ÷ 40.00 = 2.5 mol.
Moles to grams. What is the mass of 2 mol of NaOH? Multiply: 2 × 40.00 = 79.99 g.
Counting particles. 100 g contains 2.5 mol × 6.022 × 1023 = 1.506 × 1024 formula units.
Making a solution. A 0.1 mol/L solution contains 0.1 × 40.00 = 4 g of NaOH in each litre. For 250 mL, weigh 0.9999 g and make it up to the mark with water.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 4 g |
| 0.5 mol | 20 g |
| 1 mol | 40 g |
| 2 mol | 79.99 g |
| 5 mol | 200 g |
| Mass | Moles |
|---|---|
| 1 g | 0.025 mol |
| 10 g | 0.25 mol |
| 100 g | 2.5 mol |
| 1 kg | 25 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of NaOH?
The molar mass of sodium hydroxide (NaOH) is 40.00 g/mol, or 39.997 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of NaOH?
100 ÷ 40.00 = 2.5 mol.
What is the mass of one formula unit of NaOH?
Divide the molar mass by the Avogadro constant: 40.00 ÷ 6.022 × 1023 = 6.642 × 10-23 g.
What is the percentage of sodium in NaOH?
Sodium makes up 57.48% of the mass of NaOH: 1 × 22.990 ÷ 39.997 × 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.