Free Molarity Calculator
Calculate molar concentration (mol/L) from moles and volume, or rearrange the formula to solve for moles or volume when you know the molarity.
Enter any two values — the third is calculated from M = moles ÷ liters.
M = n ÷ V · n = M × V · V = n ÷ M. Use liters for volume (250 mL = 0.25 L) and the total solution volume, not the solvent alone.
Quick answer
Molarity is the number of moles of solute divided by the volume of solution in liters: M = moles ÷ volume(L). Enter any two of the three quantities — moles, volume, or molarity — and the calculator solves for the missing one. For example, 0.5 mol of solute dissolved in 2 L of solution gives a molarity of 0.25 mol/L.
Formula & method
Molar concentration
M = n ÷ V
- M — Molarity (molar concentration) in mol/L, also written as M
- n — Amount of solute in moles (mol)
- V — Volume of the total solution in liters (L)
Rearranges to n = M × V (moles) and V = n ÷ M (volume). Volume is the final solution volume, not the volume of solvent added.
Examples
- Input
- 0.5 mol of NaCl dissolved in 2 L of solution
- Result
- 0.25 mol/L
- Why
- M = n ÷ V = 0.5 mol ÷ 2 L = 0.25 mol/L. The solution is 0.25 molar (0.25 M).
- Input
- Molarity = 0.1 mol/L, Volume = 0.5 L
- Result
- 0.05 mol
- Why
- Rearrange to n = M × V = 0.1 mol/L × 0.5 L = 0.05 mol of solute are dissolved in the 0.5 L of solution.
- Input
- 2 mol of solute at a target concentration of 0.5 mol/L
- Result
- 4 L
- Why
- Rearrange to V = n ÷ M = 2 mol ÷ 0.5 mol/L = 4 L. You must dilute the 2 mol of solute up to a total of 4 L to reach 0.5 M.
- Input
- 0.25 mol of glucose in 250 mL of solution
- Result
- 1 mol/L
- Why
- Convert 250 mL to 0.25 L (÷1000), then M = n ÷ V = 0.25 mol ÷ 0.25 L = 1 mol/L. The result is a 1 M solution.
When to use this tool
- Preparing a solution of a known concentration in a chemistry, biology, or pharmacy lab and you need the moles or volume required.
- Checking the concentration of a stock solution before performing a dilution (often paired with the C1V1 = C2V2 dilution rule).
- Solving textbook or homework problems that give you two of moles, volume, and molarity and ask for the third.
- Scaling a recipe or buffer up or down while keeping the same molar concentration.
Common mistakes
- Using milliliters instead of liters. Molarity is per liter, so divide milliliters by 1000 first (250 mL = 0.25 L) or your answer will be 1000× too small.
- Measuring the volume of solvent rather than the final solution. Molarity uses the total volume after the solute is dissolved and topped up, not the amount of water you started with.
- Confusing grams with moles. Molarity needs moles, so first convert mass to moles by dividing grams by the molar mass (g/mol) before using this calculator.
- Mixing up molarity (mol/L) with molality (mol/kg of solvent). They are different concentration units and are not interchangeable, especially at high concentrations or temperatures.
Frequently asked questions
What is the formula for molarity?
Molarity equals moles of solute divided by the volume of solution in liters: M = moles ÷ liters. Rearranged, moles = molarity × liters and liters = moles ÷ molarity.
What units does molarity use?
Molarity is measured in moles per liter (mol/L), abbreviated M and read as 'molar'. A 1 M solution contains 1 mole of solute per liter of solution. Always convert volumes to liters before calculating.
How do I calculate molarity from grams?
First convert grams to moles by dividing the mass by the substance's molar mass (g/mol), then divide the moles by the volume in liters. For example, 58.44 g of NaCl is 1 mol, so 58.44 g in 2 L is 0.5 mol/L.
What is the difference between molarity and molality?
Molarity is moles of solute per liter of solution (mol/L), while molality is moles of solute per kilogram of solvent (mol/kg). Molarity changes slightly with temperature because volume expands; molality does not.
Is molarity the same as concentration?
Molarity is one specific type of concentration — the molar concentration in mol/L. 'Concentration' is a broader term that also covers mass/volume, percent, molality, and parts per million, so molarity is a subset of concentration.
How do I find the volume needed for a given molarity?
Rearrange the formula to volume = moles ÷ molarity. For example, to dissolve 2 mol of solute at 0.5 mol/L you need 2 ÷ 0.5 = 4 liters of total solution. This calculator does the rearrangement for you when you leave the volume field blank.
Sources & references
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