# Vapor pressure, BP elevation, FP depression

#### kasho11

##### Full Member
So I had a question regarding the colligative property formulas. Kaplan uses these equations without the Van't Hoff Factor, whereas TPR shows these equations with the Van't Hoff Factor included, for example osmotic pressure for Kaplan is MRT, whereas for TPR it's MiRT. I feel safer going with TPR's formula but why the disparity?

On a related note I had been struggling with the concept of vapor pressure for a while but I ended up realizing it's basically the same thing as FP depression and BP elevation. All 3 act to keep solutes in the liquid phase. The addition of more solute acts as little chemical anchors tying them down to the liquid phase and preventing them from escaping.

#### MrNeuro

##### Full Member
7+ Year Member
So I had a question regarding the colligative property formulas. Kaplan uses these equations without the Van't Hoff Factor, whereas TPR shows these equations with the Van't Hoff Factor included, for example osmotic pressure for Kaplan is MRT, whereas for TPR it's MiRT. I feel safer going with TPR's formula but why the disparity?

On a related note I had been struggling with the concept of vapor pressure for a while but I ended up realizing it's basically the same thing as FP depression and BP elevation. All 3 act to keep solutes in the liquid phase. The addition of more solute acts as little chemical anchors tying them down to the liquid phase and preventing them from escaping.

use TPRs

##### MD
Moderator Emeritus
10+ Year Member
Finding osmotic pressure as &#928; = MRT is just assuming a non-dissociating solute and thus a Van't Hoff factor of i = 1. The equation is always &#928; = iMRT (or MiRT or however you want to arrange it.)

#### MedPR

##### Membership Revoked
Removed
Finding osmotic pressure as &#928; = MRT is just assuming a non-dissociating solute and thus a Van't Hoff factor of i = 1. The equation is always &#928; = iMRT (or MiRT or however you want to arrange it.)

It's just a shorter way of writing the formula. A similar example is "F=ma" which is technically incorrect notation. The correct way to write it is "&#931;F=ma" but it's such a common equation that we very rarely bother writing in the sigma.

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