Molality Calculator

Calculate molal concentration from moles of solute and mass of the solvent.

Supports metric and imperial mass units with bidirectional solving.

Updated August 28, 2026

CreatorFrank Zhao

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What is Molality?

Molality (also called molal concentration) measures how many moles of solute are dissolved in one kilogram of solvent. Unlike molarity, molality is defined by mass rather than volume, which means it stays accurate regardless of temperature changes. This makes it the preferred concentration unit for colligative property calculations — freezing point depression, boiling point elevation, and osmotic pressure.

The unit of molality is $mol/kg$ , sometimes written as $m$ (lowercase) or “molal.”

This calculator handles the full molality workflow: start from moles and solvent mass, or go deeper by entering solute mass and molar mass to auto-derive moles first. Every field is bidirectional — you can enter any combination of known values and the calculator solves for the rest automatically.

How to Use This Calculator

Enter moles of solute

Type the number of moles in the “Moles of solute” field. The default unit is mol, but you can choose from picomoles through moles.

Enter mass of solvent

Type the solvent mass in the “Mass of solvent” field. Default unit is kilograms, with options from micrograms to pounds.

Read the molality result

The molality field updates instantly. Derived values appear with a blue highlight to distinguish them from your manual entries.

Quick Example: NaCl in Water

Dissolve $58.44\ \text{g}$ of sodium chloride in $0.5\ \text{kg}$ of water:

n = \frac{58.44}{58.44} = 1\ \text{mol}

m = \frac{1}{0.5} = 2\ \text{mol/kg}

Tip: If you already know the moles, skip the checkbox and enter moles + solvent mass directly.

Real-World Examples

Example 1: Antifreeze in Engine Coolant

An automotive engineer needs to prepare a 50% ethylene glycol coolant. She dissolves $310.4\ \text{g}$ of ethylene glycol ( $W = 62.07\ \text{g/mol}$ ) in $1\ \text{kg}$ of water. What is the molality?

n = \frac{310.4}{62.07} = 5.0\ \text{mol}

m = \frac{5.0}{1.0} = 5.0\ \text{mol/kg}

This molality value is used in the freezing point depression formula $\Delta T_f = K_f \cdot m$ to predict how much the coolant lowers the freezing point — critical data for winter-grade antifreeze formulations.

Example 2: Reverse — Finding Solvent Mass

A biochemistry student needs $0.25\ \text{mol}$ of glucose dissolved at a target molality of $1.25\ \text{mol/kg}$ . How much water is required?

M_{\text{solvent}} = \frac{n}{m} = \frac{0.25}{1.25} = 0.2\ \text{kg}

Enter $0.25\ \text{mol}$ in Moles of solute and $1.25\ \text{mol/kg}$ in Molality, and the calculator instantly derives the solvent mass — no manual rearrangement needed.

Formula & Variables

m = \frac{n_{\text{solute}}}{M_{\text{solvent}}}

Molality = moles of solute ÷ mass of solvent (kg)

When you know solute mass and molar mass:

n_{\text{solute}} = \frac{m_{\text{solute}}}{W_{\text{solute}}}

Click “Show mass of solute and molar mass” to reveal these fields and chain both steps automatically.

Variable	Meaning	Default Unit
$m$	Molality (mol per kg of solvent)	mol/kg
$n_{\text{solute}}$	Moles of solute	mol
$M_{\text{solvent}}$	Mass of solvent	kg
$m_{\text{solute}}$	Mass of solute	g
$W_{\text{solute}}$	Molar mass of solute	g/mol

All six solve directions are supported. Enter any two known values in a relation, and the third is derived automatically. For example, entering solvent mass and molality gives you moles; entering mass of solute and molar mass gives you moles — and if solvent mass is also present, molality is computed in a single chain.

Frequently Asked Questions

What is the difference between molality and molarity?

Molarity (M) is moles of solute per liter of solution. Molality (m) is moles of solute per kilogram of solvent.

The practical difference: molarity changes with temperature because solution volume expands or contracts, while molality stays constant because it depends on mass. For experiments involving significant temperature changes — such as freezing point depression or boiling point elevation — molality is the more reliable measure.

When should I use this calculator instead of a molarity calculator?

Use this calculator when your experiment or calculation involves:

Colligative properties (freezing point depression, boiling point elevation, osmotic pressure)
Work at non-ambient temperatures where solution volume may change
Industrial or analytical procedures that specify concentration in molal units

What mass units are supported?

The calculator supports micrograms, milligrams, grams, decagrams, kilograms, drachms, ounces, and pounds for both solute and solvent mass. Molar mass is in grams per mole. Moles range from picomoles to moles. You can mix units freely — the calculator handles all conversions internally.

Limitations

All input values must be positive — zero or negative masses and moles are not valid in molality calculations.
Molality assumes a pure solvent. If your solution contains multiple solutes, each contributes independently to the total molality.
Results are computed with high precision. For practical laboratory work, round to the number of significant figures justified by your input measurements.

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