Density and freezing point

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  • #1
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.
 

Answers and Replies

  • #2
gravenewworld
1,127
25
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?
 
  • #3
pari786
29
0
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?
 
  • #4
pari786
29
0
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?
 
  • #5
pari786
29
0
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
 
  • #6
pari786
29
0
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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