Asked by A

Something that will lower the freezing point of water to -3.7 degrees Celsius if one mole were placed in 1000 grams of water? To solve this I know I have to use the equation Tf= IKfm, Tf= 3.7, but how I go about the problem there if I don't know Kf or the the mole of solute (for molality)? Please show work, thank you!!
Answered by DrBob222
I think you must assume some things.
1. Since you know the solvent is water you can look up the Kf value of 1.86/mol.
2. You have your choice of solute so you can pick one that has an i = 1 or an i of some other value.
3. So 3.7 = iKm
if you chose i = 2 you could pick a solute of NaCl.
K is 1.86; therefore,
3.7 = 2*1.86m
m = about 0.99 but you need to confirm that and obtain a more accurate number if you wish.
m = mols/kg solvent or
mol = m*kg solvent = 0.99* 1 kg = 0.99 mols
mol = grams/molar mass or
grams = mols x molar mass
grams NaCl = mols NaCl (0.99) x molar mass = ?
That will get you grams NaCl to provide 3.7 delta T in 1 kg H2O.
Answered by A
Instead of NaCl, can I pick any other salt, as long as it has a Van't Hoff factor/ i value of 2?
Answered by DrBob222
Of course. van't Hoff factors are at best just good guesses; you might be closer to reality by picking something with i = 1; i.e., a complete non-electrolyte like ethyl alcohol or something like that.
Answered by A
How did you get .99 for the molality value?
There are no AI answers yet. The ability to request AI answers is coming soon!