I'm starting to learn about colligative properties and i want to know if there are different equations for boiling point elevation and freezing point depression? In the book they give me the equation: (delta)Tb= Kb times molality.

Question

But my teacher gave us: delta Tb = Kb times # of ions x # of moles.
(This is for boiling point elevation)
Which one is correct?

Answer 1

Yes and no.
The general formula for the boiling point elevation is delta T = i*Kb*m but text books often omit the i(the number of particles) since covalent compounds have i = 1. The book is right, in a limited sense, but your teacher is right in all situations. For freezing point depression,
delta T = i*Kf*m