Sodium Hydroxide Molarity Calculator

A sodium hydroxide molarity calculator works out the molar concentration of a NaOH solution from the mass of solute, the volume of water, and (optionally) the purity of the NaOH pellets. You enter the grams of NaOH, the volume in liters or milliliters, and the calculator returns the molarity in mol/L (M) along with the mass of NaOH needed to make a target molarity. This is the standard formula used in every chemistry lab for preparing NaOH solutions, and the calculator below does the arithmetic for you.

Formula

The molarity (M) of a solution is defined as the number of moles of solute per liter of solution. For sodium hydroxide, the formula is:

Molarity (mol/L) = (mass of NaOH in grams × purity fraction) / (molar mass of NaOH × volume in liters)

Where:

• Mass of NaOH (g) – the amount of solid NaOH you dissolve.
• Purity fraction – the decimal form of the purity percentage (e.g., 98% = 0.98). NaOH pellets are rarely 100% pure because they absorb water and carbon dioxide from the air.
• Molar mass of NaOH = 40.00 g/mol (Na = 22.99, O = 16.00, H = 1.01).
• Volume (L) – the final volume of the solution, not the volume of water you added.

The calculator uses the molar mass as a constant. If you need more precision, you can use 39.997 g/mol, but 40.00 is standard for most lab work.

Worked Example

Using the default values from the calculator: 4 g of NaOH, 1 L of solution, and 98% purity.

1. Convert purity to a fraction: 98% = 0.98.
2. Calculate the actual mass of NaOH: 4 g × 0.98 = 3.92 g.
3. Divide by the molar mass: 3.92 g ÷ 40.00 g/mol = 0.098 mol.
4. Divide by the volume: 0.098 mol ÷ 1 L = 0.098 mol/L.

The calculator rounds to three decimal places, so the displayed result is 0.098 M. If you used 100% purity, the result would be 0.100 M.

Why Purity Matters

NaOH pellets are hygroscopic, meaning they absorb moisture from the air. A freshly opened bottle might be 98–99% pure, but after prolonged storage it can drop to 90% or less. This is why the calculator includes a purity field. If you skip it, you will overestimate the actual molarity. For precise work, chemists often standardize a NaOH solution by titrating it against a primary standard like potassium hydrogen phthalate (KHP). The calculator gives you a good estimate, but for exact concentrations you should always titrate.

How to Use the Calculator

There are two common use cases. The first is finding the molarity of a solution you already made. Enter the mass of NaOH you dissolved, the final volume, and the purity, and the calculator gives you the concentration. The second is preparing a solution of a target molarity. Use the second output field, which tells you how many grams of NaOH you need to dissolve in the given volume to make a 1 M solution. For other target molarities, multiply that number by the desired molarity. For example, to make 0.5 M NaOH in 1 L with 98% purity, you need 0.5 × 40.00 / 0.98 = 20.41 g.

Common Mistakes

• Using water volume instead of final volume. When you dissolve NaOH in water, the final volume is slightly larger than the water you started with. Always use a volumetric flask and fill to the mark, then use that volume in the calculation.
• Forgetting the purity factor. This is the most frequent error. If you use 4 g of 98% NaOH and assume it is 100%, you get 0.100 M instead of 0.098 M – a 2% error.
• Mixing units. Volume must be in liters. If you measure in milliliters, divide by 1000 first. The calculator expects liters, so a 500 mL solution should be entered as 0.5 L.
• Confusing molarity with molality. Molarity is moles per liter of solution; molality is moles per kilogram of solvent. This calculator gives molarity.

Safety and Handling

Sodium hydroxide is a strong base and highly corrosive. Always wear gloves and safety goggles when handling pellets or solutions. Dissolving NaOH in water is exothermic – the solution gets hot, sometimes enough to boil. Add NaOH to water slowly while stirring, never add water to NaOH. Work in a well-ventilated area because the dissolution can release aerosols. If you spill a solution, neutralize it with a dilute acid like vinegar before cleaning up. Store NaOH in an airtight container because it absorbs CO₂ and moisture from the air, which changes its purity and thus its effective molarity.

Limits of the Formula

The formula assumes the NaOH is the only solute and that the solution is at room temperature. Molarity changes slightly with temperature because the volume of the solution expands or contracts. The calculator also assumes the purity is uniform, but in reality pellets may have a crust of sodium carbonate (Na₂CO₃) that forms from CO₂ absorption. For most educational and routine applications, the result is accurate enough. For analytical chemistry, you must standardize the solution by titration, as the purity of NaOH cannot be known exactly without analysis.

Frequently Asked Questions

How do I make a 1 M NaOH solution?
Dissolve 40.00 g of pure NaOH in enough water to make 1 L of solution. If your NaOH is 98% pure, you need 40.00 / 0.98 = 40.82 g. Always add NaOH to water, not the reverse, and use a volumetric flask for accuracy.

Why is my NaOH solution weaker than I calculated?
The most likely reasons are that the NaOH absorbed moisture (so you weighed less than you thought), the solution was not made to the final volume, or the purity was lower than expected. Even fresh pellets are not 100% pure. Titration against a primary standard is the only way to know the exact concentration.

Can I use this calculator for KOH or other bases?
No, because the molar mass is different. Potassium hydroxide (KOH) has a molar mass of 56.11 g/mol. You can adapt the formula by replacing 40.00 with the correct molar mass, but this calculator is specifically for NaOH.

What is the difference between molarity and normality for NaOH?
For NaOH, normality equals molarity because each mole of NaOH provides one mole of hydroxide ions (OH�). So a 0.1 M NaOH solution is also 0.1 N. This is not true for bases like calcium hydroxide, Ca(OH)₂, which provides two OH� ions per formula unit.

Does temperature affect the molarity?
Yes, slightly. Solutions expand when heated, so the same amount of solute in a fixed volume becomes less concentrated at higher temperatures. The effect is small for most lab work, but for precise measurements you should use a solution at the temperature indicated on the volumetric flask (usually 20°C).

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in m | Mass of NaOH | g | 4 | 0
in v | Volume of solution | L | 1 | 0
in p | Purity of NaOH | % | 98 | 0
out molarity | Molarity | mol/L | (m * (p / 100)) / (40.00 * v) | 3
out mass_needed | Mass for 1 M | g | (1 * 40.00 * v) / (p / 100) | 2