What is the freezing point of a solution containing 6.5 % KCl by mass (in water).?

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Answer 1 · 2018-05-12T13:05:59

Well what is the molal cryoscopic constant for water?

This site says #1.853 *K*kg*mol^-1#

And so we must work out the molality of a #6.5%# solution....we take a mass of #1*kg# of solution....

#"Molality"="Moles of solute"/"Kilograms of solvent"=((65.0*g)/(74.55*g*mol^-1))/((1000*g-65.0*g)xx10^-3*kg*g^-1)=0.933*mol*kg^-1#...

And of course there is a catch. Potassium chloride undergoes chemical REACTION in water to give two equivs of ions...

#KCl(s) stackrel(H_2O)rarrK^+ +Cl^-#

...where each ion is aquated by several water molecules...

And so there are TWO equiv of ion in solution per equiv of salt...

Finally, we get....

#0.933*mol*kg^-1xx2xx1.853*K*kg*mol^-1=3.46*K#

And thus #"fusion point"_"solution"=(0.0-3.46)^@C=-3.46*^@C#