Freezing point depression constant value clarification.

1. Feb 23, 2012

LexLuthor

Hi all,

I just need a little clarification here, please. I'm a tutor and I've been helping a student with their freezing point depression lab and the student has to determine the molar mass of an unknown solute from the experimental data provided. I understand the process and have been able to work all of the equations and explain them to the student without any problems except for one. When the final calculation is made to determine the molar mass of the unknown solute it comes out negative because the Kf provided is negative. That makes sense since it's a freezing point depression constant. However, it does not make sense for a molar mass to be negative. My question is when using the Kf value provided should it just be the absolute value of the Kf? That would make sense because then the molar mass would come out positive as we would expect a molar mass value to be. Any clarification that can be provided will be appreciated. Thanks for taking the time to read this.

- Lex

2. Feb 25, 2012

Bipolarity

$$\Delta T_{f} = K_{f}m$$

$\Delta T_{f}$ and $K_{f}$ are both negative, but the molar mass is still positive, since it is the ratio of two negative numbers.