Some Basic Concepts of Chemistry

Chemistry · Class 11

Simulation · Chemistry · Class 11

Molarity drifts with temperature, molality does not

From the lesson Reactions in solutions: concentration terms in Some Basic Concepts of Chemistry. Change the values and watch what happens.

Molarity drifts with temperature, molality does notChemistry · Class 11

The idea behind it

NCERT § "Reactions in Solutions"

  • Mass percent = (mass of solute / mass of solution) × 100.
  • Mole fraction of a component = its moles / total moles of all components; the mole fractions of all components add up to 1.
  • Molarity (M) = moles of solute per litre of solution; it changes with temperature because volume changes.
  • Molality (m) = moles of solute per kilogram of solvent; it does not depend on temperature because it uses masses only.
  • Dilution keeps the moles of solute fixed, so M₁V₁ = M₂V₂.
  • Converting molarity to molality needs the solution's density: find the mass of 1 L of solution, subtract the solute's mass to get the solvent's mass, then divide the moles of solute by that mass in kg.
  • Converting molality to molarity goes the other way: mass of solution = mass of solvent + mass of solute, then use density to get the volume.