The answer is 4.9 moles of salt will be needed to add to 1 kg of water to change the boiling point by 5 °C.
What is Boiling Point Elevation ?
Boiling point elevation is the phenomenon that occurs when the boiling point of a liquid (a solvent) is increased when another compound is added, such that the solution has a higher boiling point than the pure solvent.
Boiling point elevation occurs whenever a non-volatile solute is added to a pure solvent.
ΔT = i * Kb * m
where m, Kb and i are the molality of solution, ebullioscopic constant and Van't Hoff factor respectively
The data given in the question is ΔT = 5 °C
mass of solvent = 1 kg
Kb = 0.51 °C/(mol/kg)
Van't Hoff Factor for NaCl is 2
Substituting the values in the above equation
[tex]\rm 5 = \rm\dfrac{2\times0.51\times moles \;of \;salt}{1}[/tex]
[tex]\rm moles \;of \;salt = \rm\dfrac{5}{2\times0.51}[/tex]
moles of salt = 4.9
Therefore 4.9 moles of salt will be needed to add to 1 kg of water to change the boiling point by 5 °C.
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