Molality
The molal unit is not used nearly as frequently as the molar unit and is used in thermodynamic calculations where a temperature independent unit of concentration is needed. A molality is the number of moles of solute dissolved in one kilogram of solvent. The term molality and molarity should not be confused. While expressing the Molality it is represented by a small “m,” whereas molarity is represented by an upper case “M.”
In case of preparation of molar solution except water all other solvent must be weighed. The water is exempted from weighing because; one liter of water has a specific gravity of 1.0 and weighs one kilogram. So one can measure out one liter of water and the solute could be directly added to it. But other solvents might have a specific gravity greater than or less than one. Therefore, one liter of any solvent other than water is not likely to occupy a liter of space.
For example to make a one molal aqueous (water) solution of sodium chloride (NaCl), measure out one kilogram of water and add one mole of the solute, NaCl to it. The atomic weight of sodium is 23 and the atomic weight of chlorine is 35. Therefore the formula weight for NaCl is 58. So 58 grams of NaCl could be dissolved in 1kg water for preparation of 1 molal solution of NaCl.
QUESTIONS
1. ……………… is the unit used in thermodynamic calculations where a temperature independent unit of concentration is needed.
2. A molality is the number of moles of solute dissolved in ……………… kilogram of solvent
(a) 3 (b) 2 (c) 1 (d) 6
3. Molality it is represented by a ………………
(a) Small ‘m’ (b) Capital ‘M’ (c) Small ‘n’ (d) Capital ‘N’
