Molarity Calculator
Enter values and click Calculate.
n = m / Mrc = m / VM = n / V = m / (Mr × V)
Solution concentration depends on both the amount of dissolved solute and the final volume occupied by the complete solution. The same number of solute particles produces a higher molarity in a smaller volume and a lower molarity after additional solvent increases the volume.
The Molarity Calculator converts solute mass into moles and then relates those moles to solution volume. It reports the amount of solute, mass concentration in grams per liter, and molarity in moles per liter.
What the Molarity Calculator Finds
Moles: The chemical amount of solute, calculated from its measured mass and molar mass.
Mass concentration: The solute mass divided by the final solution volume, normally expressed in g/L.
Molarity: The number of moles of solute in one liter of solution, expressed as mol/L or M.
These values describe the same prepared solution from different perspectives. Mass concentration tracks grams per volume, while molarity accounts for molecular or formula mass.
Molarity Calculator Formulas
First, calculate the amount of solute:
Moles = Solute mass ÷ Molar mass
Calculate mass concentration using:
Mass concentration = Solute mass ÷ Final solution volume
Calculate molarity using:
Molarity = Moles ÷ Final solution volume
The two molarity steps can be combined:
Molarity = Solute mass ÷ (Molar mass × Final solution volume)
For the direct formula, use mass in grams, molar mass in g/mol, and volume in liters. The result is mol/L, numerically equal to molarity in M.
Understanding the Inputs
Solute mass: Enter the mass of the chemical substance being dissolved, not the combined mass of solute and solvent.
Molar mass: Use the molar mass of the exact chemical formula. Hydrated and anhydrous salts have different molar masses, as do an ion and the full compound that contains it.
Solution volume: Enter the final volume after the solute has dissolved and the solution has been brought to its intended mark. This is not necessarily equal to the volume of solvent originally added.
How to Use the Calculator
- Measure or enter the solute mass in grams.
- Enter the solute’s molar mass in grams per mole.
- Enter the final solution volume in liters.
- Click Calculate.
- Review the moles, mass concentration, and molarity results.
- Check the step-by-step section to confirm the substituted values.
If the available volume is in milliliters, convert it before a manual calculation:
Volume in liters = Volume in milliliters ÷ 1000
Worked Example
Suppose 10.0 g of a solute with a molar mass of 50.0 g/mol is dissolved and the final solution volume is adjusted to 2.00 L.
Calculate the amount of solute:
Moles = 10.0 ÷ 50.0 = 0.200 mol
Calculate the mass concentration:
Mass concentration = 10.0 ÷ 2.00 = 5.00 g/L
Calculate molarity:
Molarity = 0.200 ÷ 2.00 = 0.100 mol/L
The prepared solution contains 0.200 mol of solute, has a mass concentration of 5.00 g/L, and has a molarity of 0.100 M.
Use Final Solution Volume, Not Solvent Volume
Molarity is defined using the total volume of the finished solution. Adding 1 L of water to a solute does not always produce exactly 1 L of solution because dissolving and mixing can change the final volume.
In laboratory preparation, the solute is commonly dissolved in less than the target volume and then additional solvent is added until the bottom of the meniscus reaches the calibration line of a volumetric flask. The marked volume is the value used in the molarity calculation.
Molarity vs Mass Concentration
Mass concentration measures grams of solute per liter. Molarity measures moles of solute per liter. Two solutions can have the same g/L value but different molarities if their solutes have different molar masses.
For example, 10 g/L of a 50 g/mol substance corresponds to 0.20 M, while 10 g/L of a 200 g/mol substance corresponds to only 0.05 M. The heavier formula unit produces fewer moles from the same mass.
Purity and Hydrate Corrections
The calculator assumes the entered mass is entirely the stated solute. If a reagent is not pure, first find the usable mass:
Pure solute mass = Sample mass × (Purity percentage ÷ 100)
Also confirm whether the reagent is hydrated. Copper sulfate pentahydrate, for example, includes water molecules in its formula and therefore has a different molar mass from anhydrous copper sulfate.
Common Unit Relationships
1 L = 1000 mL
1 M = 1 mol/L
1 M = 1000 mM
1 mM = 0.001 M
Keep full precision through intermediate steps and round only the final reported result to reduce rounding error.
Common Mistakes
- Using solvent volume: enter the final volume of the complete solution.
- Leaving volume in milliliters: convert mL to L before using the base formula.
- Using the wrong molar mass: match the exact compound, ion, or hydrate.
- Entering total solution mass: the mass field is for the solute.
- Ignoring reagent purity: adjust the usable solute mass when purity is below 100%.
- Confusing M with mM: millimolar values differ from molar values by a factor of 1000.
Assumptions and Limitations
The calculation assumes accurate mass, molar mass, and final-volume inputs. It does not calculate molar mass from a chemical formula or automatically correct for purity, hydration state, temperature-dependent volume, density, measurement uncertainty, incomplete dissolution, or chemical reaction after mixing.
For laboratory, medical, pharmaceutical, industrial, environmental, or safety-critical preparation, follow the required protocol and verify concentration using suitable calibrated equipment and analytical methods.
Molarity Calculator FAQs
What is the formula for molarity?
Molarity equals moles of solute divided by the final solution volume in liters: M = n ÷ V.
How do I calculate molarity from grams?
Divide grams by molar mass to find moles, then divide those moles by the final solution volume in liters.
Is molarity the same as mol/L?
Yes. A concentration of 1 M means one mole of solute per liter of solution.
Should I use the volume of water added?
No. Use the final volume of the complete solution after the solute has dissolved and the solution has reached its intended volume.
Can molarity change with temperature?
Yes. Molarity depends on solution volume, and liquid volume can change with temperature even when the amount of solute remains constant.
Why is molar mass needed?
Molar mass converts the measured solute mass into moles, allowing mass-based data to be expressed as molar concentration.
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