Molality and the von Hoff factor.

[Bipolarity]

I have seen in my textbook that most colligative properties are defined in terms of molality. This must mean that there is some relation between molality and the von Hoff factor, since von Hoff factor is directly related to the colligative properties.

However, as far as I know, the only difference between molarity and molality is that molarity is measured in terms of volume of solution, whereas molality is measured in terms of kilograms of solvent.

So where does the von Hoff factor come in when defining molality? If it doesn't play a role in defining molality, then my book could have used molarity without being incorrect?

Thanks.

BiP

 

[Borek]

There is no relation between molality and Van 't Hoff factor - and it is not needed.

Colligative properties change with concentration of whatever objects are dissolved. Concentration type doesn't have to be a molality, but they are often related to temperature change - molality doesn't change with temperature change so it is more convenient than for example molarity.

However, when you dissolve 1 mole of substance in 1 kg of solvent concentration of objects created is not necessarily 1 mole/kg, this is where the Van 't Hoff factor comes into play - on top of molality, and not being part of it.

 

[DrDu]

I suppose molality is chosen because it is easier to control than molarity. Weighting substances is always more precise than volumetric measurements.

 

[Bipolarity]

I see! Thanks for the reply Borek and DrDu!
Now that I think of it in terms of temperature, it all makes sense to use molality, since masses never change.

BiP