How to calculate molality?
Molality (m) is defined as the number of moles of solute per kilogram of solvent. To calculate molality, you need to know the number of moles of solute and the mass of the solvent. Here are the steps to calculate molality:
- Determine the number of moles of solute: You can use the formula n = m/M, where n is the number of moles of solute, m is the mass of solute in grams, and M is the molar mass of the solute in grams per mole.
- Determine the mass of the solvent: Weigh the solvent (in kilograms) that will be used in the solution.
- Calculate the molality: Use the formula m = n/s, where m is the molality, n is the number of moles of solute determined in step 1, and s is the mass of the solvent in kilograms.
Here's an example:
Suppose you dissolve 5 grams of glucose (C6H12O6) in 100 grams of water.
- Determine the number of moles of glucose: Molar mass of glucose = 180.16 g/mol n = m/M = 5g / 180.16 g/mol = 0.02776 mol
- Determine the mass of the solvent: Mass of water = 100 g = 0.1 kg
- Calculate the molality: m = n/s = 0.02776 mol / 0.1 kg = 0.2776 mol/kg
Therefore, the molality of the glucose solution is 0.2776 mol/kg.