Molarity Calculator
Molarity is the most common unit of concentration in chemistry, expressing the number of moles of solute dissolved in exactly one liter of solution. This calculator lets you quickly determine solution concentration from the amount of solute and the total volume, making it essential for laboratory preparation and reaction planning.
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Formula
M = n / V
M is molarity in moles per liter (mol/L). n is the number of moles of solute. V is the total volume of solution in liters. Molarity differs from molality — it divides moles by volume of solution, not mass of solvent — so temperature changes that affect volume will also affect molarity.
How to use the Molarity Calculator
- 1
Enter your moles of solute
Value should be in mol.
- 2
Enter your volume of solution
Value should be in L.
- 3
Read your results instantly
Results update in real time as you type.
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What is molarity and why does it matter?
Molarity (symbol M) is the number of moles of a solute dissolved per liter of solution. It is the standard unit for expressing concentration in most laboratory and industrial chemistry settings. When a chemist says a solution is "2 M NaCl," they mean there are 2 moles of sodium chloride in every liter of that solution. Molarity is used when preparing reagents, diluting stock solutions, calculating reaction yields, and interpreting spectroscopic data. Because reactions happen between particles — not grams — working in moles per liter allows stoichiometric calculations to remain straightforward. A firm grasp of molarity is essential before moving on to dilutions, titrations, or equilibrium constants.
How to calculate moles from grams
If you start with a mass of solute rather than a mole count, you first need to convert grams to moles using the molar mass of the compound. Molar mass is the sum of atomic masses for all atoms in the formula, expressed in g/mol. For example, sodium chloride (NaCl) has a molar mass of approximately 58.44 g/mol. Dissolving 5.844 g of NaCl gives you 0.100 mol, so dissolving it in 0.500 L of water yields a 0.200 M solution. Always measure the final volume of solution — not just the volume of water added — because adding solute changes the total volume slightly. Use a volumetric flask for precision work.
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Common mistakes and unit pitfalls
The most frequent error when using molarity is confusing milliliters with liters. If a problem states 250 mL, you must convert to 0.250 L before dividing. Similarly, "concentration" can mean different things in different contexts — normality, molality, and mass percent are all distinct from molarity. Another pitfall is assuming that mixing volumes is additive: 500 mL of one solution plus 500 mL of another does not always yield exactly 1.000 L, especially with concentrated acids or when significant solvation occurs. For accurate laboratory work, always make solutions up to volume in a calibrated volumetric flask and allow them to equilibrate to room temperature before taking a final reading.
Tips & Insights
Convert mL to L first
Divide your volume in milliliters by 1000 before entering it here. Forgetting this conversion is the most common source of a 1000-fold error in molarity calculations.
Measure to volume, not to mass
Always make solutions to a defined final volume using a volumetric flask — do not simply add a fixed volume of solvent to the solute, as this ignores the volume change caused by dissolution.
Temperature matters for accurate work
Molarity is temperature-dependent because volume changes with temperature. For high-precision applications, calibrate volumetric glassware at the temperature you will be using and record the solution temperature.
Worked Examples
Preparing 0.5 M NaOH
Molarity = 0.5 M — dissolve 0.5 mol (20 g) of NaOH in enough water to make 1 liter of solution.
Small-scale reaction
Molarity = 0.1 M — 0.025 mol of solute in 250 mL gives the same concentration as 0.1 mol in 1 L.
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Frequently Asked Questions
What is the difference between molarity and molality?
Molarity is moles of solute per liter of solution, while molality is moles of solute per kilogram of solvent. Molality is preferred when temperature varies because it does not depend on volume.
Can molarity be greater than 1?
Yes. Many stock solutions exceed 1 M. Concentrated hydrochloric acid is about 12 M and concentrated sulfuric acid is about 18 M.
How do I convert grams to moles?
Divide the mass in grams by the molar mass of the compound in g/mol. The molar mass equals the sum of all atomic masses in the chemical formula.
Does dissolving a solute change the total volume?
Yes, adding a solute generally changes the solution volume slightly. Always prepare solutions to the final target volume in a volumetric flask rather than assuming volumes are additive.
Why is molarity written as M?
The uppercase M is standard shorthand for mol/L (moles per liter). You may also see it written as mol·L⁻¹ in some journals. The two notations mean the same thing.
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