What’s the difference between molarity and molality

Molarity is concentration of a substance that is calculated as the number of moles of solute that is dissolved in 1 liter of solution  

The symbol for molarity is M, while that for molality is m (sometimes written as –m or m to distinguish it from mass).

The formula for molarity is moles / liter while the formula for molality is moles / kg.

Molality, also called molal concentration, is a measure of the concentration of a solute in a solution in terms of amount of substance in a specified amount of mass of the solvent. This contrasts with the definition of molarity which is based on a specified volume of solution.