Freezing point depression calculations

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SUMMARY

The discussion focuses on the calculations involved in determining the molar mass of solutes using freezing point depression. The user calculated a freezing point depression (delta T) of 1.6 degrees Celsius, using a freezing point of 11.0 degrees Celsius for cyclohexanol and a cryoscopic constant (kf) of 39.4 degrees Celsius/m. The calculated molality was 0.041, leading to an erroneous molar mass of approximately 2000 g, while the correct choices were 214.4 g, 242.5 g, and 270.5 g. The user was advised to verify the freezing point and kf values for cyclohexanol, as they appeared inconsistent with known data.

PREREQUISITES
  • Understanding of colligative properties, specifically freezing point depression
  • Familiarity with the equation for molality and its application
  • Knowledge of molar mass calculation from mass and moles of solute
  • Basic laboratory skills for measuring temperature and mass accurately
NEXT STEPS
  • Verify the freezing point and kf value for cyclohexanol using reliable chemical databases
  • Practice calculations involving freezing point depression with different solutes
  • Explore the impact of experimental errors on molar mass determination
  • Learn about the significance of accurate measurements in laboratory experiments
USEFUL FOR

Chemistry students, laboratory technicians, and educators involved in teaching colligative properties and molar mass calculations will benefit from this discussion.

physicsman2
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Homework Statement


Hi, I have a problem with the data and calculations part in my lab. For some reason, I can't get a molar mass of the given solutes to match my answer. I'll provide all the equations and my numbers. Please I really need help with this

Homework Equations


deltaT = m(kf)
moles of solute=(molality)(kg of solvent used)
molar mass = mass of solute/moles of solute

The Attempt at a Solution


My freezing point for my cyclohexanol solvent was 11.0 degrees C and the freezing point I got for my solution was 9.4 degrees C(It could've been less, but my instructor told me to stop there). I found delta T to be 1.6 by subtracting the two temperatures. Then, I found the molality of the solution by dividing the change in temperature by kf, which was give to be 39.4 degrees C/m. I got .041 and went on. Next, they asked to find the moles of solute which was the molality I found to be .041 times the kg of solvent used. In grams, the solvent weighed 9.2 g, which is .0092 kg. I multiplied those two numbers and got .00038 moles. Then, they asked to find the molar mass of the solute by dividing the mass of the solute in grams, .75 g, by the moles of solute, where i got about 2000g. The only choices were 214.4g, 242.5g, and 270.5g. I can't see what I did wrong.
 
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Melting point for cyclohexanol is not 11 but 25.4 deg C and kf of 39.4 seems to be quite high - are you sure about these numbers and solvent identity?

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methods
 
i knew the freezing point for cyclohexanol was a lot higher, but that's what i got in my experiment. As for kf, they gave us that value. I'm not sure if I have to convert it or something, that's why i gave the units. My masses were all in grams but i converted the mass of my solvent to find the moles of the solute. My instructor said it's okay if our answers are off as long as we got a correct answer with the data we had. Can you check that I at least did it right? Thanks for responding
 
I got slightly below 2k as well.

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Thanks so much, i'll just have to explain the faulty experimental values
 

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