Molality Calculator
Calculate molality, moles of solute, solvent mass, solute mass, freezing point depression, and boiling point elevation with m = n / kg solvent, ΔTf = iKfm, and ΔTb = iKbm.
Select a calculation, then enter the known values.
Molality
Formula used: m = n / mass of solvent (kg)
0.5 m
Molality
500 mmol/kg
Millimolal equivalent
0.5 mol
Solute amount
1 kg
Solvent mass
Interactive Solvent & Solute Visual
A live view of solute particles dissolved per kilogram of solvent.
Step-by-Step Molality Calculation
Here's exactly how this answer was calculated, one step at a time.
Given: moles = 0.5 mol, solvent mass = 1 kg, molality = 0.5 m
Step 1: Choose the molality relationship
Molality is the amount of dissolved solute per kilogram of solvent (not solution).
m = n / mass of solvent (kg)Step 2: Convert the entered units
n = 0.5 mol; solvent mass = 1 kg; m = 0.5 mol/kgStep 3: Substitute the known values
m = 0.5 / 1Step 4: Report the result
Molality = 0.5 m
Calculated result:
0.5 m
Molality Calculator: Find Molal Concentration Online
This free molality calculator works out molality, moles of solute, mass of solvent, solute mass, and even freezing point depression or boiling point elevation, all in one tool. It is built for chemistry students, lab assistants, teachers, and anyone who needs to prepare or check a molal solution without doing the unit conversion by hand. Type in your values in mol, mmol, kg, or grams, pick what you want to find, and the calculator shows the answer along with the full working.
Molality is a concentration unit that chemistry students meet early and use often, especially in questions about colligative properties. Unlike molarity, molality does not change with temperature because it is based on the mass of the solvent, not the volume of the solution. This makes it a better choice whenever a solution is heated, cooled, or measured at different temperatures during an experiment.
What Is Molality?
Molality, written as a lowercase m, tells you how many moles of solute are dissolved in one kilogram of solvent. For example, if you dissolve 1 mole of table salt in 1 kg of water, you get a 1 molal (1 m) solution. Notice that this is different from molarity, which is based on the volume of the final solution, not the mass of the solvent alone.
Because molality depends only on mass, it does not shift when the temperature of a solution rises or falls. Volume expands with heat, so molarity can change slightly with temperature, but the mass of a solvent stays the same. This is one reason molality is the standard unit used in freezing point depression, boiling point elevation, and other colligative property questions.
Molality Formula: m = n / mass of solvent (kg)
The molality formula is simple: m = n / kg of solvent. Here, m is molality in mol/kg, n is the number of moles of solute, and kg of solvent is the mass of the solvent only, not the solution. For example, dissolving 0.25 mol of sugar in 0.50 kg of water gives a molality of 0.50 mol/kg, or 0.50 m.
You can rearrange this formula to solve for other unknowns. To find moles, use n = m × kg solvent. To find the mass of solvent needed, use kg = n / m. This calculator switches between these three forms automatically depending on which value you want to find, and it shows every step so you can follow the working or check your own homework answer.
How to Calculate Molality From Grams
When the solute is measured in grams instead of moles, first convert grams to moles using the solute's molar mass: moles = grams ÷ molar mass (g/mol). Then divide that by the mass of the solvent in kilograms to get molality. Written as one equation, this is m = mass ÷ (molar mass × kg of solvent).
For example, dissolving 5.844 g of sodium chloride, which has a molar mass of 58.44 g/mol, gives 0.100 mol of NaCl. Dissolving that in 1.000 kg of water gives a 0.100 m solution. The solute mass mode on this calculator does this in reverse: pick a target molality, a solvent mass, and a molar mass, and it tells you exactly how many grams to weigh out.
Molality vs Molarity: What Is the Difference?
Molality and molarity sound alike but measure different things. Molarity (M) is moles of solute per litre of final solution, while molality (m) is moles of solute per kilogram of solvent. Molarity depends on the total solution volume, which can shift with temperature. Molality depends only on solvent mass, which does not change when the temperature changes.
For dilute water-based solutions at room temperature, molarity and molality are often close in value, because one litre of water weighs close to one kilogram. But they are not the same thing, and the gap grows for concentrated solutions, non-aqueous solvents, or solutions studied at different temperatures. Always check which unit a question or lab procedure asks for before you calculate, and never assume 1 M equals 1 m without checking the solvent density.
Freezing Point Depression: ΔTf = i × Kf × m
Adding a solute to a solvent lowers its freezing point. This effect is called freezing point depression, and it is calculated with ΔTf = i × Kf × m. Here, ΔTf is the drop in freezing point, i is the van't Hoff factor (the number of particles the solute breaks into), Kf is the cryoscopic constant of the solvent, and m is the molality of the solution.
For water, Kf is 1.86 °C·kg/mol. So a 1 m solution of a non-ionic solute like sugar (i = 1) in water lowers the freezing point by 1.86 °C, from 0 °C down to about -1.86 °C. This is the same idea behind salting icy roads: dissolved salt particles lower the freezing point of water so ice melts at a lower temperature. Pick the freezing point mode above to try this with different solvents and van't Hoff factors.
Boiling Point Elevation: ΔTb = i × Kb × m
The opposite effect happens at the boiling point: dissolving a solute raises it. This is boiling point elevation, calculated with ΔTb = i × Kb × m, where Kb is the ebullioscopic constant of the solvent. For water, Kb is 0.512 °C·kg/mol, which is much smaller than Kf, so boiling point changes are usually less noticeable than freezing point changes for the same solution.
For example, adding salt to boiling water raises its boiling point only slightly, by a fraction of a degree for typical kitchen amounts. In a lab setting, this equation is more useful for finding the molar mass of an unknown solute: measure the boiling point rise of a solution with a known mass of solute and solvent, then work backwards through the formula to find the solute's molar mass.
What Is the Van't Hoff Factor?
The van't Hoff factor, written as i, tells you how many particles one unit of solute breaks into when it dissolves. A molecule that does not split, such as glucose or sucrose, has i = 1. An ionic compound that fully separates into ions changes the count: sodium chloride splits into Na+ and Cl-, so i = 2, while calcium chloride splits into one Ca2+ ion and two Cl- ions, so i = 3.
In real solutions, ion pairing means the actual i is often a little less than this ideal number, especially at higher concentrations. For classroom problems, the ideal value based on the number of ions in the formula is normally what is expected unless the question states an experimental value. Always check whether your solute is molecular (i = 1) or ionic (i equal to the number of ions) before using the freezing or boiling point modes.
How to Prepare a Molal Solution
To make a solution of known molality, first weigh out the correct mass of solvent, since molality is based on solvent mass rather than final solution volume. Weigh the solvent on a balance, then calculate and weigh the solute mass needed using the formulas above. Add the solute to the weighed solvent and stir or swirl until it fully dissolves.
Unlike molarity preparation, you do not need a volumetric flask calibrated to a fixed volume, because you are not trying to hit an exact final volume. You are only trying to hit an exact mass ratio between solute and solvent. This makes molality convenient for freezing point and boiling point experiments, where temperature changes during the experiment would otherwise throw off a volume-based measurement.
Common Mistakes When Calculating Molality
The most common mistake is dividing moles of solute by the mass of the whole solution instead of the mass of the solvent alone. Molality only uses solvent mass. If a problem gives you the total solution mass, you may need to subtract the solute mass first to find the solvent mass before you calculate.
Another common error is mixing up grams and kilograms. Since the molality formula needs solvent mass in kilograms, always convert grams to kilograms first by dividing by 1000. For freezing and boiling point problems, forgetting the van't Hoff factor for ionic solutes is also a frequent mistake; leaving i at 1 for a salt like NaCl will give an answer that is half of the correct value.
Real-Life and Laboratory Uses of Molality
Molality shows up in antifreeze formulas, where a fixed mass of ethylene glycol is mixed into a fixed mass of water to protect a car engine from freezing at low winter temperatures. It is used in road salting calculations, in cryobiology when protecting living cells during freezing, and in determining unknown molar masses through freezing point or boiling point experiments in a chemistry lab.
Because molality does not shift with temperature, it is also the preferred unit in thermodynamics and physical chemistry, where solutions are studied across a range of temperatures. Any time an experiment involves heating or cooling a solution and precise concentration still matters, molality is usually the more reliable unit to work with compared with molarity.
Molality Calculator FAQ and Quick Reference
Use m = n / kg solvent to find molality, n = m × kg solvent to find moles, and kg = n / m to find solvent mass. To find solute mass for a target molality, use mass (g) = m × kg solvent × molar mass. For temperature shift questions, use ΔTf = iKfm for freezing point depression and ΔTb = iKbm for boiling point elevation.
This free molality calculator is meant for study, homework checking, and general lab planning. Always double-check units, especially kilograms versus grams, and confirm the van't Hoff factor for ionic solutes before using the freezing or boiling point modes. For safety-critical or graded lab work, verify your final numbers against your course material or lab manual.
Frequently Asked Questions
What is the formula for molality?
Molality equals moles of solute divided by kilograms of solvent: m = n / kg solvent.
How is molality different from molarity?
Molality uses the mass of the solvent in kilograms, while molarity uses the volume of the final solution in litres. Molality does not change with temperature; molarity can.
How do I calculate molality from grams?
Divide grams of solute by its molar mass to get moles, then divide by the solvent mass in kilograms.
What is the van't Hoff factor?
It is the number of particles one unit of solute produces when it dissolves: 1 for molecular solutes like sugar, 2 for NaCl, 3 for CaCl2.
How do you find freezing point depression?
Use ΔTf = i × Kf × m, where Kf is the solvent's cryoscopic constant, i is the van't Hoff factor, and m is molality.