To prepare a solution of sodium hypochlorite you need the grams, and grams is molarity × volume in litres × 74.438769 g/mol. A litre of 1 M takes 74.43877 g; 500 mL of 0.1 M takes 3.7219 g; 250 mL of 0.01 M takes 186.1 mg. Enter the strength you want and the volume you want it in, and the calculator returns the mass to weigh together with the same figure in milligrams and as a percentage strength.
The figure comes from the formula. Sodium hypochlorite is 22.9898 (Na) + 35.45 (Cl) + 15.999 (O), which is 74.438769 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Chlorine makes up the largest share of the mass: 1 of the 3 atoms in a formula unit is chlorine, and they account for 35.45 g of the 74.43877 g, or 47.62% by mass. The derivation table below breaks the whole molecule down element by element.
Order of operations matters more than precision here. Dissolve the solid in rather less than the final volume, then make up to the mark once it has all gone into solution and come back to room temperature — warm solutions read low when they cool. If the calculated mass is under about 20 mg, prepare ten times the strength and dilute it tenfold by pipette instead, because C₁V₁ = C₂V₂ carries less error than a marginal weighing.
Household bleach, sold as a percentage of active chlorine. It decomposes steadily, so an old bottle is weaker than its label says. At 74.43877 g/mol it is heavier than 3 of the 34 salts covered here, and that ranking matters more than it looks: sodium nitrite has a molar mass of 68.99477 g/mol, so a gram of it contains 7.89% more formula units than a gram of sodium hypochlorite. Weigh by mass and you are not weighing equal amounts of substance.
Purity is the usual gap between the calculation and the balance. A reagent sold at 98% means 1.4888 g of a nominal 74.43877 g is something else, so for exact work you divide the weighed mass by the assay figure on the certificate. For most purposes the difference is smaller than the error in reading the meniscus, but it is systematic rather than random, so it does not average out.
The formula
c- The molarity you want, in mol/L
V- The volume you want to make, in millilitres
74.438769- The molar mass of sodium hypochlorite in g/mol, derived from its formula
m- The mass of sodium hypochlorite to weigh out, in grams
How it works, step by step
- Enter the molarity you want, in mol/L.
- Enter the volume you want to make, in millilitres.
- The two are multiplied together and by 74.438769 g/mol.
- The result is the mass to weigh; dissolve it in part of the volume and then make up to the mark.
Worked examples
250 mL of 0.1 M sodium hypochlorite
Weigh 1.86097 g. That is 0.1 M × 0.25 L × 74.43877 g/mol, and it comes to 1860.97 mg if your balance is set to milligrams. Dissolve in about 175 mL first, then make up to 250 mL.
500 mL of 0.5 M sodium hypochlorite
Weigh 18.6097 g. That is 0.5 M × 0.5 L × 74.43877 g/mol, and it comes to 18,609.7 mg if your balance is set to milligrams. Dissolve in about 350 mL first, then make up to 500 mL.
1000 mL of 1 M sodium hypochlorite
Weigh 74.4388 g. That is 1 M × 1 L × 74.43877 g/mol, and it comes to 74,438.8 mg if your balance is set to milligrams. Dissolve in about 700 mL first, then make up to 1000 mL.
How to read your score
Frequently asked questions
What is the molar mass of sodium hypochlorite (NaClO)?
It is 74.438769 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 22.9898 (Na) + 35.45 (Cl) + 15.999 (O). One mole of sodium hypochlorite therefore weighs 74.43877 g, and a gram of it is 13.434 mmol.
What percentage of sodium hypochlorite is chlorine?
47.62% by mass. Each formula unit contains 1 chlorine atom contributing 35.45 g of the 74.43877 g total, so 100 g of sodium hypochlorite contains 47.62 g of chlorine and a kilogram contains 476.23 g of it.
How much sodium hypochlorite do I need for 1 litre of 1 M solution?
74.438769 g — the molar mass in grams, which is what a 1 molar solution means. For 1 L of 0.1 M it is 7.44388 g, and for 1 L of 0.01 M it is 0.744388 g.
How much for 100 mL of 0.1 M?
0.744388 g. The volume is a tenth of a litre and the strength a tenth of molar, so the mass is a hundredth of 74.43877 g. In milligrams that is 744.388 mg.
Do I dissolve the solid first or make up the volume first?
Dissolve first, in perhaps 70% of the final volume, then make up to the mark. Adding solid to a full flask overshoots the volume and leaves the solution weak, and undissolved solid at the mark means the strength keeps changing as it goes in.
Can I dilute a stock solution instead of weighing?
Yes, and it is more accurate for small amounts. C₁V₁ = C₂V₂: to get 500 mL of 0.01 M from a 0.5 M stock, take 10 mL of stock and make up to 500 mL. Weighing the 372.19 mg that 500 mL of 0.01 M needs directly is the harder of the two.
Solution recipes for sodium hypochlorite
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Chlorine (Cl) | 1 | 35.45 | 35.45 | 47.62% |
| Sodium (Na) | 1 | 22.98977 | 22.98977 | 30.88% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 21.49% |
| Total — one mole of sodium hypochlorite | 74.438769 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 74.438769 g/mol.
| Target | for 100 mL | for 250 mL | for 500 mL | for 1 L |
|---|---|---|---|---|
| 0.001 M | 7.44388 mg | 18.6097 mg | 37.2194 mg | 74.4388 mg |
| 0.005 M | 37.2194 mg | 93.0485 mg | 186.097 mg | 372.194 mg |
| 0.01 M | 74.4388 mg | 186.097 mg | 372.194 mg | 744.388 mg |
| 0.05 M | 372.194 mg | 930.485 mg | 1.86097 g | 3.72194 g |
| 0.1 M | 744.388 mg | 1.86097 g | 3.72194 g | 7.44388 g |
| 0.15 M | 1.11658 g | 2.79145 g | 5.58291 g | 11.1658 g |
| 0.2 M | 1.48878 g | 3.72194 g | 7.44388 g | 14.8878 g |
| 0.25 M | 1.86097 g | 4.65242 g | 9.30485 g | 18.6097 g |
| 0.5 M | 3.72194 g | 9.30485 g | 18.6097 g | 37.2194 g |
| 1 M | 7.44388 g | 18.6097 g | 37.2194 g | 74.4388 g |
| 2 M | 14.8878 g | 37.2194 g | 74.4388 g | 148.878 g |
Each cell is molarity × volume in litres × 74.438769 g/mol. Dissolve the solid first and then make up to the marked volume — adding solid to a full volume overshoots it.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Ammonium chloride | NH4Cl | 53.489 | 18.695 |
| Sodium chloride | NaCl | 58.43977 | 17.112 |
| Sodium nitrite | NaNO2 | 68.99477 | 14.494 |
| Sodium hypochlorite (this page) | NaClO | 74.43877 | 13.434 |
| Potassium chloride | KCl | 74.5483 | 13.414 |
| Ammonium nitrate | NH4NO3 | 80.043 | 12.493 |
| Sodium bicarbonate | NaHCO3 | 84.00577 | 11.904 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.