boling point elevation

[Genie133]

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when do you use: [change in] T=Kb i m vs. T=Kb m ??? how do you know when to use which? or can you always use T=Kb m?

i ran into a problem like this:

"Student prepares a solution with 1500 g of H2O and and unkown amount of CaCl2. The boiling point is elevated to 101.56 degrees. What is the mass of CaCl2? (Kb=0.52 C/m)"

So basically, using T=Kb m, you get 1.56=0.52 x m, so m=3, but since there are 3 moles of ions( 1 Ca and 2 Cls), it's really 1 molal. and you can use that to figure out the mass of CaCl2.

how does that compare to something like this:

"What is the boiling point temperature of a 0.5 m solution of Na2SO4 dissolved in water at 1atmosphere? (Kb = 0.512 C/m)"

T= 0.512 x 0.5, T=.256, so the boiling point temp is 100.256.

What i'm stuck on here is where the i term would figure into both of these problems, because I don't know when to use it and when not to. Help!

 

[ChubbyChaser]

If im not mistaken always use kbMi


In that clase CaCl2 would dissoicate into three ions so i would be 3,and in the second one two Na ions and one So4 ion, So i woul dbe 3. Correct?

 

[changarang]

the way i understand it, the formulas for boiling point elevation and freezing point depression should always incorporate the i. that is, the change in temperature depends on the number of dissolved particles, not simply the concentration (molality) of the solution.

so i agree with the solution to the first problem, in which i = 3 because CaCl2 is a soluble salt and so every unit of CaCl2 gives 3 units of ions (1 Ca2+ and 2 Cl-), but i disagree with the answer to the second problem. in that case, the i should also equal 3 because Na2SO4 is also a soluble salt, and so every unit of Na2SO4 gives 3 units of ions (2 Na+ and 1 SO42-). so instead of delta T = 0.512 x 0.5 = 0.256, i would say delta T = 0.512 x 0.5 x 3 = 0.384.

in short, you're always supposed to consider i when calculating delta T. the only time it wouldn't make a difference is if you're dissolving something that doesn't break up into more ions, in which case the i = 1.

the tricky part is if instead of calculating delta T, you have to calculate the mass. then you have to do the reverse calculation, as you already pointed out. so in the CaCl2 problem, there are 3 molal of dissolved solutes, but only 1 molal of CaCl2, and the 1 molal is the number you need to use when figuring out the mass.

 

[Genie133]

the way i understand it, the formulas for boiling point elevation and freezing point depression should always incorporate the i. that is, the change in temperature depends on the number of dissolved particles, not simply the concentration (molality) of the solution.

so i agree with the solution to the first problem, in which i = 3 because CaCl2 is a soluble salt and so every unit of CaCl2 gives 3 units of ions (1 Ca2+ and 2 Cl-), but i disagree with the answer to the second problem. in that case, the i should also equal 3 because Na2SO4 is also a soluble salt, and so every unit of Na2SO4 gives 3 units of ions (2 Na+ and 1 SO42-). so instead of delta T = 0.512 x 0.5 = 0.256, i would say delta T = 0.512 x 0.5 x 3 = 0.384.

in short, you're always supposed to consider i when calculating delta T. the only time it wouldn't make a difference is if you're dissolving something that doesn't break up into more ions, in which case the i = 1.

the tricky part is if instead of calculating delta T, you have to calculate the mass. then you have to do the reverse calculation, as you already pointed out. so in the CaCl2 problem, there are 3 molal of dissolved solutes, but only 1 molal of CaCl2, and the 1 molal is the number you need to use when figuring out the mass.

that's what i thought at first (about the 2nd problem), but kaplan gives the solution as .256, so I don't know how to reconcile that with the knowledge that delta T = Kb i m ...

 

[changarang]

kaplan is wroooooooooooong!!

(no, but seriously...i think kaplan is wrong. if kaplan is right, then they need to come up with a good explanation because that answer just totally goes against what i learned in chem. not that the world revolves around what i learned in chem. hehe.)

 

[xrevision]

Yea, boiling point elevation is a colligative property so it depends on the number of particles in the solution and not the concentration.