What is the molality of my solution?
Type the solute mass and its molar mass, or its moles, and the mass of the solvent. The molality calculator gives moles of solute per kilogram of solvent, or the solute mass to weigh out for a molality you want.
- Answer
- 17.883 mol/kg
The answer is 17.883 mol/kg.
- Molality (mol/kg)
- 17.883
- Solute (mol)
- 35.7661
- Solute (g)
- 2,220
- Molality (mmol/kg)
- 17,883
Answer: 17.883 mol/kg. The answer is 17.883 mol/kg.
How to calculate
Computes molality, moles of solute per kilogram of solvent, from the solute mass and molar mass or its moles, or works out the solute mass needed for a target molality.
Example with the default inputs (Find Molality from mass, Solute mass 2,220 g, Molar mass, g/mol 62.07, Solvent mass 2,000 g): The answer is 17.883 mol/kg.
Method: molality = moles of solute ÷ kilograms of solvent; moles = grams ÷ molar mass. For a target molality: moles = molality × kg of solvent; grams = moles × molar mass.
- The solvent mass is the solvent alone, not the solution. Molality does not change with temperature, because it uses masses, not volumes.
Worked examples
Each example is checked against the calculator on every build.
- Find Molality from mass, Solute mass 2,220 g, Molar mass, g/mol 62.07, Solvent mass 2,000 g gives Molality, mol/kg 17.883035, Answer 17.883 mol/kg.Source: OpenStax, Chemistry 2e, §11.4 Colligative Properties (molality m = mol solute ÷ kg solvent), https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties, Example 11.3 Calculating Mole Fraction and Molality: 17.9 m
- Find Solute mass, Molality, mol/kg 0.5, Molar mass, g/mol 58.44, Solvent mass 250 g gives Solute (g) 7.305, Solute, mol 0.125, Answer 7.305 g of solute (0.125 mol).Source: OpenStax, Chemistry 2e, §11.4 Colligative Properties (molality m = mol solute ÷ kg solvent), https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties
- Find Molality from moles, Solute, mol 0.3, Solvent mass 600 g gives Molality, mol/kg 0.5, Molality (mmol/kg) 500.Source: OpenStax, Chemistry 2e, §11.4 Colligative Properties (molality m = mol solute ÷ kg solvent), https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties
How it works
Molality. moles of solute n = solute grams ÷ molar mass (g/mol), or the moles you type. Then
molality = n ÷ kilograms of solvent (mol/kg, written m).
Solute mass. n = molality × kilograms of solvent, and grams of solute = n × molar mass.
Masses convert exactly: 1 g = 0.001 kg; 1 mg = 10⁻⁶ kg; 1 lb = 0.45359237 kg; 1 oz = 0.028349523125 kg. The rows also show the molality in mmol/kg (× 1,000) and, when a molar mass is known, the solute grams.
Exact arithmetic. Each value is read as the exact decimal you typed, in its unit, so each result is an exact fraction until it is rounded for display.
Output format. The answer line shows … mol/kg, or … g of solute (… mol), each number to 6 significant figures with no thousands separators. The rows show 6 significant figures; values of 10¹⁵ or more, or below 10⁻⁶, show in scientific form.
When there is no answer. A molality over 10⁹ mol/kg, or a value too small to hold as a number.
Assumptions
- The solvent mass is the solvent alone.
Worked examples by hand
Antifreeze (OpenStax Example 11.3). n = 2,220 ÷ 62.07 = 35.7661 mol; m = 35.7661 ÷ 2.000 = 17.8830 mol/kg, shown as 17.883 (the book: 17.9 m).
Salt for a 0.500 m solution. n = 0.500 × 0.250 = 0.125 mol; mass = 0.125 × 58.44 = 7.305 g.
From moles. 0.3 mol in 0.6 kg: m = 0.3 ÷ 0.6 = 0.5 mol/kg = 500 mmol/kg.
Other questions people ask
What is molality?
Molality (m) is the moles of solute per kilogram of solvent: m = mol solute ÷ kg solvent. A 1 m solution has one mole of solute for each kilogram of solvent.
How is molality different from molarity?
Molarity divides by the liters of solution; molality divides by the kilograms of solvent alone. Volumes change with temperature and masses do not, so molality stays the same when a solution warms or cools.
How do I calculate molality from grams?
Divide the solute grams by its molar mass to get moles, then divide by the solvent mass in kilograms. 2,220 g of ethylene glycol (62.07 g/mol) in 2.00 kg of water is 35.77 mol ÷ 2.00 kg = 17.9 m.
How much solute do I need for a given molality?
Multiply the molality by the kilograms of solvent to get moles, then by the molar mass. A 0.500 m salt solution in 250 g of water needs 0.125 mol, or 7.305 g of NaCl.
Should I use the mass of the solution or of the solvent?
The solvent. If you know only the solution mass, subtract the solute mass first.
Where is molality used?
In colligative properties: the boiling point rise and freezing point drop of a solution are proportional to its molality, as in ΔT_f = K_f × m.