Molar mass of sodium hydroxide
The molar mass of sodium hydroxide (NaOH) is 39.997 g/mol: the sum of the atomic weights 22.990 + 15.999 + 1.0080. School problems round it to 40 g/mol (Na = 23, O = 16, H = 1).
Breakdown by element
M(NaOH) = 22.990 + 15.999 + 1.0080 = 39.997 g/mol
The result is rounded to the precision of the least precise atomic weight. Atomic weights from IUPAC (CIAAW). Agrees with PubChem: 39.997 g/mol. PubChem CID 14798
What is sodium hydroxide?
Sodium hydroxide (caustic soda, lye) is a white solid and a strong alkali. It burns skin and eats through fabric, so it is handled with gloves and safety glasses.
In air NaOH absorbs water and carbon dioxide. It is used to make soap and paper and to clear blocked drains, and it is produced by electrolysis of sodium chloride solution.
Properties
- State at 25 °C
- Solid
- Density
- 2.13 g/cm³ 25 °C
- Melting point
- 323 °C
- Boiling point
- 1388 °C
- Solubility in water
- soluble
Sources: PubChem (HSDB, NIOSH), Wikipedia (CRC Handbook), NIST Chemistry WebBook; each value agrees in two of them.
Molar mass problems with sodium hydroxide
For school problems (Na = 23, O = 16, H = 1): M(NaOH) = 40 g/mol
Problem 1
How many moles are in 120 g of NaOH?
n = m ÷ M = 120 g ÷ 40 g/mol
Answer: 3 mol
Problem 2
What is the mass of 1.5 mol of NaOH?
m = n × M = 1.5 mol × 40 g/mol
Answer: 60 g
Problem 3
What is the mass percentage of Na in NaOH?
w(Na) = M(Na) ÷ M(NaOH) = 23 ÷ 40 = 0.575
Answer: 57.5%
Problem 4
How many OH⁻ ions are in 0.1 mol of NaOH?
N = n × NA = 0.1 mol × 6.02 × 10²³ mol⁻¹
Answer: 6.02 × 10²²
Common substances
What is molar mass?
Molar mass M is the mass of one mole of a substance, about 6.022 × 10²³ of its particles. It is measured in grams per mole (g/mol).
To find it, add up the atomic weights of all the atoms in the formula: multiply each element’s atomic weight by its subscript, the number of its atoms. In g/mol, the molar mass equals the relative molecular mass Mr.
For water: M(H₂O) = 2 × 1.0080 + 15.999 = 18.015 g/mol.