Molarity is the concentration of a solution expressed as moles of solute per litre of solution, with units of mol/L, usually abbreviated to M. A 1 M solution contains one mole of solute in every litre. Because a mole of every substance contains the same number of particles, molarity tells you directly how many particles are present, which is what reaction stoichiometry actually depends on — unlike a percentage, which tells you only about mass.

How do I calculate molarity?

Divide the amount of solute in moles by the volume of solution in litres: C = n ÷ V. If you have a mass rather than moles, convert first by dividing by the molar mass, which gives the combined form C = m ÷ (MW × V). Dissolving 9 g of sodium chloride, molar mass 58.44, in enough water to make 1 litre gives 9 ÷ 58.44 = 0.154 mol/L — physiological saline.

How much solute do I need for a given molarity?

Multiply concentration by volume by molar mass: m = C × V × MW. For 250 mL of 0.5 M glucose, that is 0.5 × 0.25 × 180.156 = 22.52 g. Weigh that out, dissolve it in rather less than 250 mL of water, then make the solution up to exactly 250 mL. Weighing accurately matters more than it might appear — a 5 percent error in mass is a 5 percent error in every result that follows.

Why is it the volume of solution, not the volume of solvent?

Because dissolving a solute changes the total volume. Sugar dissolved in water occupies space between the water molecules, so the final volume is not simply the water you added plus nothing. If you add one litre of water to 58.44 g of salt you end up with slightly more than one litre of solution and therefore slightly less than 1 M. Making the solution up to a marked line on a volumetric flask avoids the problem entirely, which is why every laboratory protocol is written that way.

What is the difference between molarity and molality?

Molarity is moles per litre of solution; molality is moles per kilogram of solvent. The distinction matters because volume changes with temperature while mass does not, so molarity drifts as a solution warms or cools while molality stays fixed. For everyday bench work at room temperature molarity is the practical choice. For work involving temperature changes, or colligative properties like freezing point depression, molality is the correct measure.

How do I dilute a solution?

Use C₁V₁ = C₂V₂. The amount of solute does not change when you add solvent, so the product of concentration and volume stays constant. To make 500 mL of 0.1 M from a 1 M stock, you need 0.1 × 0.5 ÷ 1 = 0.05 L, so take 50 mL of stock and make it up to 500 mL. Always add concentrated solution to solvent rather than the reverse, particularly with acids, because the mixing releases heat.

What does percent w/v mean, and how does it relate to molarity?

Percent weight per volume is grams of solute per 100 mL of solution, so 0.9% w/v saline is 0.9 g in 100 mL, or 9 g per litre. Converting to molarity requires the molar mass: divide grams per litre by the molar mass. For saline that is 9 ÷ 58.44 = 0.154 M. The two describe the same solution, but only molarity tells you how many particles are present, which is what determines osmotic pressure and reaction behaviour.

What are typical concentrations in everyday solutions?

Blood glucose sits at about 5 millimolar, physiological saline at 0.154 M, seawater at roughly 0.6 M in dissolved salt, and household vinegar at about 0.83 M acetic acid. Laboratory concentrated hydrochloric acid is around 12 M and concentrated sulfuric acid about 18 M. That last pair spans more than three thousand times the concentration of blood glucose, which is a useful reminder of why concentrated reagents demand respect.

Does temperature affect molarity?

Yes, though usually only slightly. Liquids expand as they warm, so the same amount of solute occupies a larger volume and the molarity falls. Water expands by roughly 2 percent between 20 and 80 degrees Celsius, so a solution prepared at room temperature is about 2 percent more dilute when hot. For most bench work that is negligible, but for precise analytical work solutions are prepared and used at a stated temperature, or molality is used instead.

Molarity Calculator (C = n / V)

Work out the concentration of a solution, or the mass you need to weigh out to make one. Enter any three of mass, molar mass, volume and concentration and this calculator returns the fourth, then tells you how to actually prepare it. Molarity is the amount of solute per litre of solution, written mo...

C = n / V

Three values entered — solving for concentration.

MASS OF SOLUTE

MOLAR MASS

VOLUME OF SOLUTION

CONCENTRATION

solved: 0.1540 M

COMMON SOLUTIONS

CONCENTRATION

0.1540 mol/L

MILLIMOLAR

154.004

AMOUNT

0.1540 mol

g PER LITRE

9.000

Mass of solute

9.0000 g

Molar mass

58.440 g/mol

Volume

1.0000 L

Amount of substance

0.15400 mol

Mass per litre

9.000 g/L

Percent w/v

0.9000 %

HOW TO PREPARE IT

Weigh out 9.0000 g of solute (molar mass 58.440 g/mol).

Dissolve it in a volume of solvent smaller than the final target.

Make the solution up to exactly 1.0000 L — the final volume is what counts, not the volume of solvent added.

The result is 0.1540 mol/L, containing 0.15400 mol of solute.

THE SOLUTION

Volume of solution, not volume of solvent. Molarity is defined against the final volume after the solute has dissolved. Dissolving a solid changes the total volume, so adding one litre of water to the solute does not give one litre of solution — which is why laboratory recipes always say “make up to” rather than “add”.

HOW CONCENTRATED IS THAT?

IF YOU DILUTE IT

Diluted 2×

0.0770 M

Diluted 5×

0.0308 M

Diluted 10×

0.0154 M

Diluted 100×

0.00154 M

From C₁V₁ = C₂V₂ — take 0.5000 L of this solution and make it up to 1.0000 L to halve the concentration.

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HOW TO USE

1
Enter any three of the four boxes — mass of solute, molar mass, volume of solution and concentration. Known values get a solid black border; the one you leave blank is outlined in gold and fills in automatically.
2
Pick units beside each field. Mass in grams, milligrams, micrograms or kilograms; volume in litres, millilitres, decilitres or microlitres; concentration in molar down to nanomolar. Everything converts to SI before the arithmetic runs.
3
Read the concentration in the black box, with millimolar, amount of substance and grams per litre alongside. The key-facts grid adds percent w/v, which is how many clinical and commercial solutions are labelled.
4
Follow the four preparation steps underneath. They tell you exactly what to weigh and, crucially, to make the solution up to the final volume rather than adding that volume of solvent.

THE FORMULAS

MolarityC = n / V

From massC = m / (MW × V)

Mass neededm = C × V × MW

Volume neededV = m / (C × MW)

Amount of substancen = m / MW

DilutionC₁V₁ = C₂V₂

Mass concentrationg/L = C × MW

One molar1 M = 1 mol/L

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Last updated: July 28, 2026 · Formula verified · Eagle-eyed accuracy for every calculation.