[Faisal]

My classmates and I have no idea how to do this question.
What is the molality of ethanol,C2H5OH, in an aqueous solution in which the mole fraction of ethanol is 0.082?

[Faisal]

oh I figured it out...change the 0.082 to 82/1000.Then find the number of moles of water by subtracting 1000 by 82 which gives 918 moles of H2O.Multiplying 918 by 18 gives the mass of water.And then find molality by dividing 82 by the mass of water.

[ARGOS++]

Dear Faisal;

Very close!,  ─   But not exactly!

But as MolaLity is defined as moles per Kilogram final Solution your MolaLity will be = Moles Alcohol divided by the mass of the whole solution!
(Maybe also your fraction needs an additional check.)

Good Luck!
                    ARGOS⁺⁺

[Faisal]

Dear Faisal;

Very close!,  ─   But not exactly!

But as MolaLity is defined as moles per Kilogram final Solution your MolaLity will be = Moles Alcohol divided by the mass of the whole solution!
(Maybe also your fraction needs an additional check.)

Good Luck!
                    ARGOS⁺⁺

yeah i have to divide the mass of water by 1000 to convert it to kg.But wht do you mean "alcohol divided by the mass of the whole solution"....definition of molality is moles of solute over mass of solvent(not solution)

[ARGOS++]

Dear Faisal;

Sorry!,  ─   Your are right, I did also a mistake!
I apologize.

Good Luck!
                    ARGOS⁺⁺

[Borek]

Take a look at conversion examples here for general methodology:

http://www.chembuddy.com/?left=concentration&right=toc

then take a look here:

http://www.chembuddy.com/?left=CASC&right=concentration-cheat-sheet