Molarity Molality Converter

Convert between molarity (M), molality (m), mole fraction, and mass percent for solutions.

Convert from

Value to convert (M, m, or %)

Molar mass of solute (g/mol) (g/mol)

NaCl=58.44, glucose=180, ethanol=46

Solution density (g/mL) — for molarity conversion (g/mL)

Water=1.00

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Why Two Concentration Scales Exist

Molarity divides by volume of solution; molality divides by mass of solvent. That distinction seems minor until temperature changes: volume expands with heat while mass does not, so molarity varies with temperature and molality does not.

M = mol solute / L solution m = mol solute / kg solvent

Property	Molarity	Molality
Denominator	Volume of solution	Mass of solvent
Temperature dependent	Yes	No
Convenient for	Titrations, volumetric work	Colligative properties, physical chemistry
Needs density to convert	—	Yes

This is why colligative property equations — freezing point depression, boiling point elevation — are always written in molality. Those experiments involve temperature changes by definition, and a volume-based concentration would shift during the measurement.

m = 1000M / (1000ρ − M × MW)

The conversion needs solution density because the two scales measure fundamentally different denominators. In dilute aqueous solution the values nearly coincide, since a litre of dilute solution weighs about a kilogram and contains almost a kilogram of water. They diverge as concentration rises.

Worked Examples

Example 1: 1.0 M NaCl (MW=58.44, d=1.04)

wp=58.44/(10×1.04)=5.62%

Result: m=1000×1/(1.04×1000-58.44)=1.019 mol/kg

Isotonic saline is ~0.154M = 0.9%

Example 2: Sea water: 3.5% NaCl by mass

m=35/(58.44×96.5/100)

Result: M from density 1.025

Seawater ionic calculations

Example 3: Why they diverge

6.0 M NaCl, density 1.20 g/mL

Result: m = 7.06 mol/kg, 18% higher than M

At high concentration the solute occupies significant volume and contributes mass, so a litre of solution contains well under a kilogram of water.

Example 4: Freezing point depression

0.5 m aqueous solution, K_f = 1.86 °C/m, i = 1

Result: ΔT_f = 0.93°C

The equation uses molality specifically. Using molarity would introduce error, since the solution contracts as it cools toward freezing.

Common Mistakes

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Using molarity for colligative property calculations

Freezing point depression and boiling point elevation require molality, because the measurement itself changes temperature and would alter a volume-based concentration.

⚠️

Confusing mass of solvent with mass of solution

Molality divides by solvent mass alone. For a 20% solution, one kilogram of solution contains only 800 g of solvent — a 25% error if confused.

⚠️

Converting without density

Molarity and molality relate through solution density, which must be measured or looked up. There is no density-free conversion.

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Assuming they are interchangeable in concentrated solutions

They agree closely only in dilute aqueous solution. At high concentration or in non-aqueous solvents they diverge substantially.

Frequently Asked Questions

Molarity vs molality?

Molarity (M=mol/L solution): temperature-dependent (volume changes with T). Molality (m=mol/kg solvent): temperature-independent (mass doesn't change). Use molality for colligative properties, thermodynamics. Use molarity for titrations, dilutions.

Dilution?

M1V1=M2V2 only works for molarity. For colligative properties (boiling point elevation ΔTb=Kbm, freezing point depression ΔTf=Kfm), molality is always used.

What is the difference between molarity and molality?

Molarity is moles per litre of solution; molality is moles per kilogram of solvent. Molarity changes with temperature because volume does, while molality does not.

Why do colligative properties use molality?

Because those measurements involve temperature changes. A volume-based concentration would shift during the experiment, whereas mass-based molality stays fixed.

Why do I need density to convert?

Because the two scales use different denominators — solution volume versus solvent mass. Density is what links them.

When are molarity and molality nearly equal?

In dilute aqueous solution, where a litre weighs about a kilogram and is almost entirely water. They diverge as concentration rises.

Formula Explorer connections

Interpretation: This formula connects solute amount and solution volume, mass or particle count to concentration and colligative behavior. Assumption: Distinguish solution volume from solvent volume, use the stated temperature, and account for dissociation or nonideal activity when required.

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