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What is molal elevation ? Explain. |
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Answer» Solution :Increase in boiling point of a solution prepared by dissolving one gram molar mass of non volatile solute in one KILOGRAM of solvent is called as molal elevation. The boiling point of a solution is always higher than that of the pure solvent in which the solution is prepared, the elevation of boiling point ALSO depends on the number of solute MOLECULES rather than their nature. Let `T_(b)^(0)` be the boiling point of pure solvent and `T_(b)` be the boiling point point of solution. The increase in the boiling point `Delta T_(b)=T_(b)-T_(b)^(0)` is known as elevation of boiling point. For dilute solutions the elevation of boiling point `(Delta _(b))` is directly proportional to the molal concentration of the solute in a solution. Thus, `Delta T_(b)prop m` `Delta T_(b)=K_(b)m ""` ....(1) Where, m molality `K_(b)=` Boiling Point Elevation Constant or Molal Elevation Constant (Ebullioscopic CONSTAT) The unit of `K_(b)` is K kg `MOL^(-1)`. If `w_(2)` gram of solute of molar mass `M_(2)` is dissolved in `w_(1)` gram of solvent, then molality, m of the solution is given by the expression : `m=(w_(2)xx M_(2))/(w_(1)//1000)=(1000xx w_(2))/(M_(2)xx w_(1))` Substituting the value of molality in equation (1) we get `Delta T_(b)=(K_(b)xx1000xx w_(2))/(M_(2)xx w_(1))` `M_(2)=(1000xx w_(2)xx K_(b))/(Delta T_(b)xx w_(1))` 
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