molarity vs molality

Discussion in 'MCAT Study Question Q&A' started by peaceandlove, May 15, 2008.

  1. peaceandlove

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    hi pls someone help me figure this out what is the difference btw molarity amd molality and how molality is more favorable. i get the formula molality=mole/kg of solvent , molarity= mole/vol of solution(L). i just cannot wrap my head around this concept.... i mean what is the diff... i keep thinking kg in terms of solids and...... i dunno , some pls help clarify.... maybe i need a breather from something. thanks
     
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  3. BloodySurgeon

    Moderator Emeritus Lifetime Donor Classifieds Approved

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    molality is uses the volume of solvent (so 1kg of H2O is 1L). molarity uses the volume of the solution which includes the solutes and solvents.

    I think you are talking about boiling point elevation and freezing point depression with molality and molarity. molality will give a higher BP for the same magnitude because the solute takes up some room so if the solvent takes up the same volume as a solvent and solute then the actual concentration is higher.
     
  4. jigga5

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    If you know the formulas there isnt much more to it is there?

    Can you give an example problem where you are stuck?
     
  5. Kaustikos

    Kaustikos Archerize It

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    MCAT is more than just knowing those formulas. It will test your knowledge about the concepts involving these two, as Bloody Surgeon has pointed outed in one example. There can be more;
    such as how molality can remain unchanged vs molarity changing in certain situations.

    If you heat a solution up, the molality never technically changes as it's still the same mass of solvent, regardless if it boils and becomes a gas. If you heat a solution in terms of molarity; the volume DOES change (bigger container) as it boils and evaporates.
     
  6. BlackSails

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    molality is moles solute per unit mass of solvent
    molarity is moles solute per unit volume of solvent
    mole fraction is moles solute per moles solvent

    Molality is used for colligative properties (freezing point depression, boiling point elevation)

    Molarity is used for reactions and equilibria (technically, the activity is used)

    Mole fraction is used for vapor pressure and raoult's law
     
  7. peaceandlove

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    thanks guys, took another look at it plus ur responses... makes sense now...
    i think i just needed to call it a night.. i had been studying literally all day. thanks!!!
     

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