Molarity vs Molality: Understanding the Difference

[nothing123]

Just having some problems understanding why one is dependent on pressure and temperature and the other is not. If molarity is moles of solute/litres of solvent, why would this change with pressure and temperature? A litre of water will remain a litre of water at any given temperature and pressure no? If somehow that changes and equating 1 litre of water to be 1 kg of water, wouldn't we be destroying mass? If that were in fact the case, wouldn't the molality be dependent on these factors too then (since 1L = 1 kg)?

Thanks.

 

[symbolipoint]

Nothing123 wonders:

A litre of water will remain a litre of water at any given temperature and pressure no? If somehow that changes and equating 1 litre of water to be 1 kg of water, wouldn't we be destroying mass?

That is wrong.

 

[GCT]

Just having some problems understanding why one is dependent on pressure and temperature and the other is not. If molarity is moles of solute/litres of solvent, why would this change with pressure and temperature? A litre of water will remain a litre of water at any given temperature and pressure no? If somehow that changes and equating 1 litre of water to be 1 kg of water, wouldn't we be destroying mass? If that were in fact the case, wouldn't the molality be dependent on these factors too then (since 1L = 1 kg)?

Thanks.

There are density changes with changes in temperature and pressure - are you not familiar with this phenomena?

 

[nothing123]

Hmm so as we increase the temperature of a solvent say water, we're giving kinetic energy to the molecules which allows them to spread out more and thus letting more solute dissolve. This would increase the molarity. However, why wouldn't the molality also increase? If more solute can dissolve in that same 1 kg of water at a higher temperature, how isn't molality also affected by temperature?

 

[Borek]

You still don't get the most important thing. Density of the solution changes with temperature, thus 1L = 1kg holds only for 4 deg C. When you heat the solution its volume changes, thus molarity changes as well. But molality is constant, as it is per mass - and mass is unchanged.

 

[nothing123]

Yes, I think I got it now. I was confusing the idea of solubility with molarity, which is simply a measurement of solubility. Thanks.

 

[Borek]

molarity, which is simply a measurement of solubility.

No, it is a measurement of concentration.

 

[Saad Ahmed]

As we know
1 molal mean 1 mole of solute dissolve in 1 kg solvent...
1 molar mean 1 mole of Solute dissolve in 1 dm3 solution
then how could we effected 1 molar solution with increase in Temperature ?

 

[Borek]

Imagine you have exactly 1L of 1M solution of NaCl at some temperature. Obviously it contains exactly 1 mole of of NaCl.

Now you heat it and the solution expands. Imagine you heated it enough so that the volume is 1.001L now (that's just 1mL more). This 1.001L of solutions still contains 1 mole of NaCl, but its molarity is now 1 mol/1.001L=0.999M.