Question about Molality and Temperaturw

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Alpha Centauri

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Mar 6, 2013
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Hey guys. So I got a question wrong on bootcamp and now that I know it is wrong, I DO remember that molality is temperature independent. However, I am having a hard time understanding why.


So it says that there is no vaporization change, so in my mind I think that the mass is constant, but why wouldnt the volume be constant? I imagine a pot of water that is being heated with an open top. If heat is added but there is no vaporization, how could there be a volume change if no liquid is evaporating?

I know that answer A and B is obviously incorrect. Density is mass / volume, and molarity is mol solute / L of solution. Molality is mol solute over kg solvent. Writing this out, I can see, from my old point of view, that C and D would be correct. However, knowing that molality is temperature independent, I still fail to see how there is a change in Liters, for molarity to no longer be constant (because no water evaporated)

After thinking about it, if you heat say 1 liter of water in a pot without vaporization, would the volume increase just because heat is added and the molecules expand from each other? In that case, would you still have 1 L of water? Maybe there would be 1.1 . Where is this mysterious water coming from?! Omg my brain right now! :rolleyes:

PS: Sorry for the title typo.
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Ari Rezaei

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Mar 14, 2012
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It's easier to think about mercury in a thermometer than a pot of boiling water. When you increase the temperature of the mercury liquid inside a thermometer, does it vaporize? No, the liquid just expands a little bit (which is the concept of thermal expansion), and as the mercury liquid expands the thermometer reveals a higher temperature. Therefore, you do have more volume. However, the mass stays the same. If the volume increases, then that means the molarity changes. However, molality is measured based on the mass, not the volume, and since the mass doesn't change, the molality stays constant.
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