Written and maintained by CASRAI Editorial Board
Last updated
The molarity of an NaOH solution is the moles of sodium hydroxide dissolved per litre of finished solution. Divide the mass of NaOH by its molar mass (39.997 g/mol) to get moles, then divide by the final volume in litres. To make a given molarity, run it backwards: mass = molarity x volume x 39.997.
How to calculate the molarity of NaOH
Molarity (M) is moles of solute per litre of solution. For sodium hydroxide, PubChem lists a molecular weight of 39.997 g/mol (computed by PubChem 2.2, release 2025.09.15; the CDC NIOSH Pocket Guide gives 40.0). The calculation takes two steps.
- Moles: n = m / 39.997, with m in grams.
- Molarity: M = n / V, with V in litres of final solution.
In one line: M = m / (39.997 x V). Units: g, g/mol, mol, L and mol/L (written M).
Molarity of NaOH is per litre of solution, not per litre of water
V is the volume of the finished solution, not the volume of water you started with. Dissolve the NaOH in part of the water, then bring it to the final volume, for example in a volumetric flask. For the general method, including dilution with C1V1 = C2V2, see the guide to molarity and solution calculations in the lab.
Molarity of NaOH: grams needed for common concentrations
Mass of NaOH needed for the final volume shown, from mass = molarity x volume x 39.997 g/mol. Values are rounded to three significant figures, so a 1 M solution rounds to 40.0 g per litre rather than using the cruder 40 g/mol shortcut.
| Target molarity | NaOH per 1 L of solution | NaOH per 250 mL of solution |
|---|---|---|
| 0.01 M | 0.400 g | 0.100 g |
| 0.05 M | 2.00 g | 0.500 g |
| 0.1 M | 4.00 g | 1.00 g |
| 0.25 M | 10.0 g | 2.50 g |
| 0.5 M | 20.0 g | 5.00 g |
| 1 M | 40.0 g | 10.0 g |
| 2 M | 80.0 g | 20.0 g |
| 5 M | 200 g | 50.0 g |
| 10 M | 400 g | 100 g |
Source for the molar mass: PubChem, Sodium hydroxide (CID 14798), checked 2026-10-29. The table values are calculated from it; they do not account for the purity of any particular reagent, so check the label of the product you use.
Worked examples
Example 1: find the molarity from a mass
4.00 g of NaOH is dissolved and made up to 500 mL of solution. Moles: 4.00 g / 39.997 g/mol = 0.1000 mol. Volume: 500 mL = 0.500 L. Molarity: 0.1000 mol / 0.500 L = 0.200 M.
Example 2: find the mass for a target molarity
You need 250 mL of 0.5 M NaOH. Moles: 0.5 mol/L x 0.250 L = 0.125 mol. Mass: 0.125 mol x 39.997 g/mol = 5.00 g, matching the 250 mL column in the table.
Calculate NaOH molarity in Excel, R or Python
With the mass in grams in A2 and the final volume in litres in B2:
Excel
=A2/39.997/B2R
mw_naoh <- 39.997
molarity <- function(mass_g, volume_l) mass_g / mw_naoh / volume_l
molarity(4.00, 0.500) # 0.2 mol/LPython
MW_NAOH = 39.997 # g/mol
def molarity(mass_g, volume_l):
return mass_g / MW_NAOH / volume_l
def mass_needed(molarity_m, volume_l):
return molarity_m * volume_l * MW_NAOH
print(round(molarity(4.00, 0.500), 4)) # 0.2
print(round(mass_needed(0.5, 0.250), 2)) # 5.0Safety: sodium hydroxide and the SDS
Sodium hydroxide is classified under the GHS with the signal word Danger and the corrosive pictogram (GHS05). The hazard statements in PubChem’s aggregated ECHA classification include H290 (may be corrosive to metals), H314 (causes severe skin burns and eye damage) and H318 (causes serious eye damage). The supplier’s safety data sheet (SDS) for the exact product you hold is the authoritative document for handling, storage and first aid. To read the labels, see GHS labels, pictograms, signal words and hazard statements.
- Exposure limits (CDC NIOSH Pocket Guide, checked 2026-10-29): NIOSH recommended exposure limit of 2 mg/m3 as a ceiling; OSHA permissible exposure limit of 2 mg/m3 as a time-weighted average; IDLH 10 mg/m3.
- First aid per NIOSH: flush skin with water immediately; irrigate eyes immediately.
- Incompatibilities per NIOSH: water, acids, flammable liquids, organic halogens, and metals such as aluminum, tin and zinc.
- Prevent skin and eye contact; NIOSH lists eyewash and quick-drench facilities as part of sanitation and protection.
More on handling and response: corrosive chemicals in the lab and chemical spill kits. For a broader view of solution preparation, see buffer and solution preparation.
FAQ
What is the molarity of NaOH?
An NaOH solution has no single molarity; it is whatever concentration you prepare. One mole is 39.997 g of NaOH, so 39.997 g in 1 L of final solution gives 1 M.
How do you calculate the molarity of NaOH?
Divide the grams of NaOH by 39.997 g/mol to get moles, then divide by the solution volume in litres.
How many grams of NaOH make 0.1 M?
About 4.00 g per litre of final solution, or 1.00 g per 250 mL, calculated from the molar mass.
Sources
- PubChem, Sodium hydroxide (CID 14798), checked 2026-10-29.
- CDC NIOSH, NIOSH Pocket Guide to Chemical Hazards: Sodium hydroxide, checked 2026-10-29.
