📌 Quick Answer
The molar mass of NaOH (sodium hydroxide) is 39.997 g/mol, which is normally rounded to 40 g/mol.
It is calculated by adding the atomic masses of its three atoms: Na (22.990) + O (15.999) + H (1.008) = 39.997 g/mol.
🔹 Key Takeaways
- Molar mass of NaOH = 39.997 g/mol ≈ 40 g/mol.
- One mole of NaOH therefore weighs about 40 grams.
- Percentage composition by mass: Na ≈ 57.5%, O ≈ 40.0%, H ≈ 2.5%.
- NaOH is a strong base widely used in acid–base titrations, soap-making, paper and water treatment.
What Is Molar Mass?
The molar mass of a substance is the mass of one mole (6.022 × 1023 formula units) of that substance, expressed in grams per mole (g/mol). For a compound, it equals the sum of the atomic masses of all the atoms in its chemical formula.
Step-by-Step Calculation of the Molar Mass of NaOH
Sodium hydroxide has the formula NaOH — one sodium atom, one oxygen atom and one hydrogen atom. Add their standard atomic masses:
| Element | Symbol | Atomic mass (g/mol) | Atoms | Contribution |
|---|---|---|---|---|
| Sodium | Na | 22.990 | 1 | 22.990 |
| Oxygen | O | 15.999 | 1 | 15.999 |
| Hydrogen | H | 1.008 | 1 | 1.008 |
| Total molar mass of NaOH | 39.997 g/mol | |||
So the molar mass of NaOH is 39.997 g/mol, rounded to 40 g/mol for everyday calculations.
Percentage Composition of NaOH
- Sodium (Na): 22.990 / 39.997 × 100 ≈ 57.5%
- Oxygen (O): 15.999 / 39.997 × 100 ≈ 40.0%
- Hydrogen (H): 1.008 / 39.997 × 100 ≈ 2.5%
Worked Examples: Grams ↔ Moles
Example 1 — Moles to grams: What is the mass of 0.25 mol of NaOH?
Mass = moles × molar mass = 0.25 × 40 = 10 g.
Example 2 — Grams to moles: How many moles are in 20 g of NaOH?
Moles = mass / molar mass = 20 / 40 = 0.5 mol.
Why the Molar Mass of NaOH Matters
The molar mass of NaOH is essential for preparing standard solutions and for stoichiometry. For example, to make 1 litre of a 1 M NaOH solution you dissolve exactly 40 g of NaOH in water and make up the volume to 1 litre. In acid–base titrations, the 40 g/mol value converts a measured mass into moles so the reaction can be balanced against an acid such as HCl.
Common Mistakes to Avoid
- Using 23 for sodium instead of 22.99 — small, but it shifts the total.
- Forgetting the hydrogen atom (treating NaOH as Na + O = 39).
- Confusing molar mass (g/mol) with mass of a sample (g).
Frequently Asked Questions
What is the molar mass of NaOH?
The molar mass of NaOH (sodium hydroxide) is 39.997 g/mol, commonly rounded to 40 g/mol.
How many grams are there in one mole of NaOH?
One mole of NaOH weighs about 40 grams, because its molar mass is 39.997 g/mol.
How do you calculate the molar mass of NaOH?
Add the atomic masses of its atoms: Na (22.990) + O (15.999) + H (1.008) = 39.997 g/mol.
Is molar mass the same as molecular weight?
For practical purposes yes. Molecular weight is a relative (unitless) number, while molar mass is the same value expressed in g/mol.
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