Molar Concentration: Formula and How to Calculate It

This calculator finds molar concentration, also called molarity, from the amount of solute and the final solution volume. Enter the solute amount in moles and solution volume in liters to get concentration in mol/L (M), plus the equivalent concentration in mol/mL.

Formula

Molar concentration is the amount of dissolved solute divided by the total volume of the solution:

C = n / V

• C is molar concentration, in moles per liter (mol/L), commonly written M.
• n is the amount of solute, in moles (mol).
• V is the final volume of the solution, in liters (L).

The calculator also converts the result to mol/mL by dividing mol/L by 1,000. This is a unit conversion, not a different concentration calculation. For example, 1 mol/L equals 0.001 mol/mL.

Worked Example

Using the calculator’s default values, the solute amount is 0.5 mol and the final solution volume is 1 L.

1. Apply the formula: C = n / V.
2. Substitute the values: C = 0.5 mol / 1 L.
3. Calculate: C = 0.5 mol/L, or 0.500 M to three decimal places.
4. Convert to mol/mL: 0.5 / 1,000 = 0.000500 mol/mL.

The first calculator result is 0.500 mol/L, and the second is 0.000500 mol/mL.

What You’ll Need

• The amount of solute in moles. If you know its mass instead, you will also need its molar mass, usually listed in g/mol.
• The final volume of the solution, measured in liters or converted to liters.
• A calculator and, for preparing a real solution, suitable laboratory equipment such as a balance and volumetric flask.

Step-by-Step

1. Identify the solute and solvent. The solute is the substance being dissolved; the solvent is the liquid or other material it dissolves in. Concentration uses the amount of solute and the volume of the complete solution, not the volume of solvent added at the start.
2. Find the solute amount in moles. If the amount is already given in mol, use it directly. If you are given mass, calculate moles using n = m / Mᵣ, where m is mass in grams and Mᵣ here means molar mass in g/mol. For instance, 10 g of a substance with molar mass 50 g/mol is 10 / 50 = 0.20 mol. Use the molar mass for the substance’s actual chemical formula, including any waters of hydration if applicable.
3. Determine the final solution volume. Use the volume after the solute has dissolved and the solution has been brought to its final volume. Convert milliliters to liters by dividing by 1,000; for example, 250 mL = 0.250 L. Convert other units before calculating.
4. Divide moles by liters. Calculate C = n / V. With 0.20 mol in 0.250 L, C = 0.20 / 0.250 = 0.80 mol/L, or 0.80 M.
5. Report the units and sensible precision. State the result in mol/L or M and retain appropriate significant figures based on the measurements. Avoid suggesting the result is more precise than the amount and volume measurements allow.
6. If making a solution, bring it to the calculated final volume. Dissolve the measured solute in less than the target volume, then add solvent until the solution reaches the target mark and mix thoroughly. Adding a target volume of solvent to the solute is not the same as making the solution up to that final volume.

Common Mistakes

• Using mL as if it were L: 250 mL is 0.250 L. Forgetting this conversion changes the answer by a factor of 1,000.
• Using solvent volume instead of solution volume: Molarity is based on the total volume after the solute is dissolved.
• Dividing mass directly by volume: grams per liter is a mass concentration, not molarity. Convert grams to moles using molar mass first.
• Choosing the wrong formula mass: Check the chemical formula, ionic form, and hydration state. A mismatch produces an incorrect mole amount.
• Rounding too early: Keep extra digits during intermediate calculations and round the final answer to match the precision of the measurements.

Tips

• Write units alongside every value. They make conversion and formula errors easier to spot.
• Check the scale of the result: dividing mol by L should produce mol/L. A larger amount in the same volume means a higher concentration; the same amount in a larger volume means a lower one.
• For a dilution with no solute added or removed, use C₁V₁ = C₂V₂. Keep each concentration and its corresponding volume paired, and use consistent volume units.
• When preparing a solution in practice, follow the relevant lab procedure and chemical safety information. Some substances require specific handling, and dissolving can change temperature or volume.

Troubleshooting

The answer seems 1,000 times too large or small: Check whether the volume was entered in liters. Convert mL to L before using the formula, or make sure the calculator’s volume field is labeled in the unit you entered.

You have grams but no mole value: Look up the correct molar mass for the exact chemical formula, then divide the mass in grams by that molar mass. If the formula or hydration state is unknown, you cannot reliably convert mass to moles yet.

Your calculated concentration does not match a label: Confirm whether the label means molarity (mol/L), mass concentration (such as g/L), or another quantity. Also check whether it describes the stock solution or a diluted mixture.

The physical solution does not reach the expected volume: Allow the solute to dissolve and the solution to return to the specified measurement temperature if the procedure requires it. Read the volumetric mark correctly and make the solution up to the mark, rather than adding a fixed volume of solvent.

The result changes noticeably with rounding: Recalculate using unrounded values and check measurement precision. If the input volume or mass is only approximate, report the concentration accordingly rather than adding unsupported decimal places.

Frequently Asked Questions

What does M mean in concentration?
M means molar, or mol/L: the number of moles of solute in each liter of solution. A 2 M solution contains 2 mol of solute per liter of solution.

Is molarity the same as molar concentration?
Yes. Molarity is the common name for molar concentration, expressed as moles per liter.

Can I calculate molarity from grams?
Yes, if you know the solute’s molar mass. Divide grams by g/mol to get moles, then divide those moles by the final solution volume in liters.

Should I use the volume of solvent or solution?
Use the final volume of the complete solution. The solute can affect the volume, so solvent volume alone is generally not the value used for molarity.

How do I find the volume needed for a target concentration?
Rearrange C = n / V to V = n / C. Keep the amount in moles and concentration in mol/L to obtain volume in liters.

Does temperature affect molar concentration?
It can. Solution volume may change with temperature, so molarity can change even if the amount of solute remains constant. Use the temperature and measurement conditions specified by the relevant procedure.

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in n | Amount of solute | mol | 0.5 | 0
in v | Solution volume | L | 1 | 0
out c | Molar concentration | mol/L | n / v | 3
out cm | Molar concentration | mol/mL | c / 1000 | 6