The key difference between molar and molal mass is that molar mass is the mass of a substance per mole, while molal mass is the mass of a substance per kilogram of solvent in a solution.
Molar vs Molal Mass
The primary difference between molar and molal mass is given below:
| Aspect | Molar Mass | Molal Mass |
| Definition | The mass of one mole of a substance, expressed in grams per mole (g/mol). | The mass of solute present in one kilogram of solvent, expressed in grams per kilogram (g/kg). |
| Symbol | M or Mm | m or mm |
| Calculation | Obtained by summing the atomic masses of all atoms in a molecule or formula unit. | Calculated by dividing the mass of the solute by the mass of the solvent in kilograms. |
| Units | grams per mole (g/mol) | grams per kilogram (g/kg) |
| Use | Used in stoichiometric calculations and relating moles to mass. | Used in colligative properties calculations, particularly in solutions and determining changes in physical properties. |
| Relation to Mole | Mass of one mole of a substance. | Mass of solute in relation to the mass of the solvent in a solution. |
| Dependence on Temperature and Pressure | Generally does not change with temperature and pressure changes. | The mass of one mole of a substance, is expressed in grams per mole (g/mol). |
| Example | The molar mass of water (H2O) is approximately 18.015 g/mol. | A molal concentration of 0.5 mol/kg means 0.5 moles of solute dissolved in 1 kg of solvent. |
What is Molar Mass?
Molar mass refers to the mass of one mole of a substance and is expressed in units of grams per mole (g/mol). It is a characteristic property of a chemical compound and is calculated by summing the atomic masses of all the atoms present in its molecular or formula unit. Molar mass is essential in various areas of chemistry, including stoichiometric calculations, determining the mass of a given number of moles of a substance, and understanding the relationships between mass, moles, and particles.
What is Molal Mass?
Molal mass, also known as molality, refers to the mass of solute (substance being dissolved) present in one kilogram of solvent (dissolving substance) in a solution. It is expressed in grams per kilogram (g/kg). Molality is used as a measure of concentration in solutions and is particularly important in the study of colligative properties, which are properties of a solvent that change when solute particles are added to it.
Molal mass can be calculated using the formula:
Molal Mass (mm) = Mass of Solute (in grams) / Mass of Solvent (in kilograms)