The molar mass of sodium hydroxide (NaOH) is 39.997 g/mol.
Case matters: CO is carbon monoxide, Co is
cobalt. Brackets and hydrates are fine — try
or
.
| Element | Atoms | Atomic mass | Mass contributed | Percent |
|---|---|---|---|---|
| Hydrogen (H) | 1 | 1.0080 | 1.008 | 2.52% |
| Sodium (Na) | 1 | 22.98976928 | 22.990 | 57.48% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 40.00% |
| Total | 3 | 39.997 | 100.00% |
1.008 + 22.990 + 15.999 = 39.997 g/mol
Grams and moles
25 g of sodium hydroxide is 0.6250 mol, because 25 ÷ 39.997 = 0.6250.
These elements have no single atomic weight. Their isotopic mix varies slightly depending on where on Earth the sample came from, so the figure used here is the conventional value IUPAC publishes for when one number is needed: hydrogen is 1.0080, and really lies between 1.00784 and 1.00811; oxygen is 15.999, and really lies between 15.99903 and 15.99977.
Sodium hydroxide comes to 39.997 g/mol, close enough to 40 that most bench work treats it as exactly that. Sodium holds 57.48% of the mass, oxygen 40.00% and the lone hydrogen just 2.52%.
The roundness is convenient and slightly treacherous. A 1 M solution needs 40 grams per litre, which is easy to remember, but solid sodium hydroxide is hygroscopic and absorbs water from the air fast enough to gain measurable mass while it sits on the balance. It also absorbs carbon dioxide, converting slowly to sodium carbonate. Pellets left open for a day will not weigh what the label implies, which is why standard solutions of it are titrated against a primary standard rather than trusted from the weighing.
Dissolving it releases a lot of heat. A litre of 1 M solution warms by roughly ten degrees, and concentrated preparations can boil if the solid is added too quickly. Always add the pellets to water, never water to the pellets.