Calculating Molality and Mass of KBr for Freezing Point Depression

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Discussion Overview

The discussion revolves around calculating the mass of KBr needed to achieve a specific freezing point depression in water. Participants explore the application of colligative properties, specifically focusing on molality and the van 't Hoff factor in the context of an online assignment.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant outlines a method to calculate the mass of KBr needed by using the freezing point depression formula and molality, initially arriving at a mass of 47.6 g.
  • Another participant questions the need to consider the van 't Hoff factor for KBr, indicating a potential oversight in their calculations.
  • It is noted that the van 't Hoff factor for KBr is 2, as it dissociates into two ions in solution.
  • A participant recalculates the mass of KBr after considering the van 't Hoff factor, arriving at a new mass of 23.9 g.
  • One participant expresses gratitude for the reminder about the van 't Hoff factor, indicating that it helped them arrive at the correct answer in their assignment.

Areas of Agreement / Disagreement

Participants generally agree on the importance of the van 't Hoff factor in the calculations, but there is no consensus on the initial calculations presented, as different values for the mass of KBr are proposed.

Contextual Notes

Participants' calculations depend on the assumption of the density of water and the correct application of the van 't Hoff factor, which may affect the final results. The discussion does not resolve the discrepancies in the initial mass calculations.

pari786
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Assume that the density of water is 1.00 g/ml . Calculate the quantity of KBr that would need to be added to lower the freezing point of 535 ml of water by 1.40 degree celsius:

What I'm trying to do is the following:

Step 1:

Tf = Kf . m

I took Tf = 1.4
Kf = 1.86
and found the molality as 0.75

Step 2:

then I used the molality equation which is

molality = moles solute/kg solvent

molality = 0.75
kg solvent = 0.535 kg
and got the no. of moles as 119 g/mol


Step 3

no. of moles = mass /molar mass and
got 47.6 g for mass

but the answer is still wrong ... I don't know where I'm making mistake. It's an online assignment so I don't know what the answer should be. But please guys help me and check if I'm doing everything right.
 
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pari786 said:
Assume that the density of water is 1.00 g/ml . Calculate the quantity of KBr that would need to be added to lower the freezing point of 535 ml of water by 1.40 degree celsius:

What I'm trying to do is the following:

Step 1:

Tf = Kf . m

I took Tf = 1.4
Kf = 1.86
and found the molality as 0.75

Step 2:

then I used the molality equation which is

molality = moles solute/kg solvent

molality = 0.75
kg solvent = 0.535 kg
and got the no. of moles as 119 g/mol


Step 3

no. of moles = mass /molar mass and
got 47.6 g for mass

but the answer is still wrong ... I don't know where I'm making mistake. It's an online assignment so I don't know what the answer should be. But please guys help me and check if I'm doing everything right.


what is the vanthoff factor for KBr?
 
gravenewworld said:
what is the vanthoff factor for KBr?

oh so you mean to say that I do have to consider vanthoff factor for KBr too in this problem?
I must have forgotten about that?
 
pari786 said:
oh so you mean to say that I do have to consider vanthoff factor for KBr too in this problem?
I must have forgotten about that?

vanthoff factor for KBr is 2 since we get two ions from one mole of KBr.
Is that right?
 
pari786 said:
vanthoff factor for KBr is 2 since we get two ions from one mole of KBr.
Is that right?

Hi, I'm doing the caculations again, see if they are right now,
I'm getting molality as 0.376
no. of moles =0.20116
molar mass =119 g/mol
mass = 23.9 g
 
hey, thanks for reminding me about the i value ... I tried it in the assignment questions and i got the right answer.
Thanks a lot for the help.
 

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