Molarity Calculator
Pick the quantity you need — molarity, grams of solute, volume of solution or molar mass — enter the other three, and get the answer with the working written out. For the common job of making a solution, it also says how to prepare it. Free, no sign-up.
Case matters: Co is cobalt, CO is carbon monoxide. Hydrates use · or *, charges use ^ (Fe^3+).
2 for H₂SO₄ as an acid, 1 for HCl, 2 for Ca(OH)₂.
Results
1.461 g
Mass of solute
0.02500 mol
Amount of solute
The working
250 mL = 0.25 L
n = c × V = 0.1 mol/L × 0.25 L = 0.025000 mol
m = n × M = 0.025000 mol × 58.44 g/mol = 1.461 g
How to make it
Weigh out 1.461 g of NaCl. Dissolve it in less than 250.0 mL of solvent, transfer it to a volumetric flask, and add solvent up to the 250.0 mL mark. The result is 0.1000 M.
How to Use the Molarity Calculator
- 1. Choose what to solve for. Molarity, mass of solute, volume of solution or molar mass.
- 2. Identify the solute. Type its formula and the molar mass is worked out for you, or switch to "A molar mass" and type it in g/mol. When you solve for the molar mass itself, no formula is needed.
- 3. Enter the other quantities with their units. Mass in µg to kg, volume in µL to L, molarity in pM to M. The answer's unit is chosen with the menu beside it.
- 4. Read the answer and the recipe. The working shows every conversion; the recipe tells you how to weigh, dissolve and make up to volume. Optional fields add normality and molality.
The Molarity Formula
Molarity
c = n / V (mol/L, written M)
Moles from mass
n = m / M (g ÷ g/mol)
Combined
c = m / (M × V)
Molarity c is the amount of solute n in moles divided by the volume of the whole solution V in litres — not the volume of solvent you started with. IUPAC calls it amount concentration; the unit is mol/L, abbreviated M. Here m is the mass of solute in grams and M is its molar mass in g/mol (not the unit M). Rearranged, the same relationship gives each quantity the calculator can solve: m = c × V × M, V = m ÷ (M × c), and M = m ÷ (c × V).
Worked Examples
Mass needed (the default inputs). How much sodium chloride makes 250.0 mL of 0.1000 M solution? The molar mass of NaCl is 58.44 g/mol. Convert the volume to 0.2500 L; then n = 0.1000 mol/L × 0.2500 L = 0.02500 mol and m = 0.02500 mol × 58.44 g/mol = 1.461 g.
Molarity from a weighed amount. 5.00 g of glucose, C₆H₁₂O₆ (180.156 g/mol), is dissolved and made up to 500.0 mL. n = 5.00 ÷ 180.156 = 0.02775 mol, and c = 0.02775 mol ÷ 0.5000 L = 0.0555 M, to three significant figures.
Volume. What volume of 2.00 M NaOH (39.997 g/mol) contains 10.0 g? n = 10.0 ÷ 39.997 = 0.2500 mol, so V = 0.2500 mol ÷ 2.00 mol/L = 0.125 L, or 125 mL.
Molar mass of an unknown. If 2.00 g of a solid makes 100.0 mL of 0.1110 M solution, M = 2.00 g ÷ (0.1110 mol/L × 0.1000 L) = 180 g/mol — consistent with glucose.
Making the Solution in the Lab
- Weigh the solute on a balance whose readability matches the precision you need, and rinse any weighing boat into the flask.
- Dissolve it in less than the final volume of solvent inside a volumetric flask; swirl until clear.
- Add solvent until the bottom of the meniscus sits on the calibration mark, then stopper and invert to mix.
- Do not add the target volume of water to the solid: the solute takes up room, so the final volume would be too large and the molarity too low.
Molarity, Molality and Normality
| Measure | Definition | Unit | Changes with temperature? |
|---|---|---|---|
| Molarity (c) | moles of solute per litre of solution | mol/L (M) | Yes, because volume does |
| Molality (b) | moles of solute per kilogram of solvent | mol/kg (m) | No, because mass does not |
| Normality (N) | equivalents of reactive species per litre of solution | eq/L (N) | Yes |
Molality is the better choice for freezing-point and boiling-point work because it does not drift as the solution warms. Normality depends on the reaction — 0.100 M H₂SO₄ is 0.200 N as an acid (two H⁺ per mole) but 0.100 N if only one proton reacts — and IUPAC discourages it; use moles and a balanced equation where you can. Enter equivalents per mole or the solvent mass in the optional fields and the calculator adds both.
Common Mistakes
- Using the solvent volume. Molarity divides by the volume of the finished solution.
- Mixing units. 250 mL must become 0.250 L before dividing; the calculator converts for you.
- Using the anhydrous molar mass for a hydrate. Weighing CuSO4·5H2O needs 249.677 g/mol, not 159.602. Type the hydrate formula; the Molar Mass Calculator handles it.
- Reporting too many digits. The answer is limited by your least precise measurement; see the Significant Figures Calculator.
Frequently Asked Questions
- How do you calculate molarity from grams and volume?
- Divide the mass by the molar mass to get moles, then divide the moles by the solution volume in litres. For 5.00 g of glucose in 500.0 mL: 5.00 ÷ 180.156 = 0.02775 mol, and 0.02775 ÷ 0.5000 = 0.0555 M.
- How many grams do I need to make a 1 M solution?
- One molar means one mole per litre, so you need one molar mass of solute per litre of final solution: 58.44 g of NaCl for 1 L of 1 M NaCl, or 14.61 g for 250 mL. Dissolve it in less water than the final volume, then make up to the mark.
- What is the difference between molarity and molality?
- Molarity is moles of solute per litre of solution; molality is moles per kilogram of solvent. For dilute aqueous solutions near room temperature they are close, but only molality stays fixed when the temperature changes.
- Why is the solution volume used instead of the solvent volume?
- Concentration describes the finished mixture. Dissolving a solute changes the total volume by an amount that depends on the substance, so the only reliable way to hit a target molarity is to make up to a calibrated volume.
- How do I convert molarity to normality?
- Multiply the molarity by the number of equivalents each mole supplies in the reaction: HCl is 1, H₂SO₄ as an acid is 2, and Ca(OH)₂ as a base is 2. A 0.100 M solution of H₂SO₄ is therefore 0.200 N for acid–base work. Normality depends on the reaction, so state which one you mean.
- How do I find the molar mass from molarity?
- Solve c = m ÷ (M × V) for M: divide the mass by the product of the molarity and the volume in litres. It is a useful check on an unknown solid, as long as you know the mass dissolved, the final volume and the concentration, for example from a titration.