Delta T is making my solution come out negative

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SUMMARY

The discussion centers on calculating the molar mass of lauryl alcohol using the freezing point depression method. The freezing point of benzene is given as 5.5 degrees Celsius, and the solution freezes at 4.1 degrees Celsius. The user initially misapplied the formula for Delta T, leading to a negative molar mass calculation. Upon clarification, it was established that Delta T should be calculated as the difference between the freezing point of pure benzene and the freezing point of the solution, confirming that Delta T = Tf^0 - Tf.

PREREQUISITES
  • Understanding of colligative properties, specifically freezing point depression
  • Familiarity with the formula for molality: molality = mol of solute/kg solvent
  • Knowledge of the freezing point depression constant (Kf) for solvents
  • Basic algebra skills for rearranging equations and solving for variables
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  • Study the concept of colligative properties in detail
  • Learn about the calculation of molar mass using freezing point depression
  • Explore the significance of Kf values for various solvents
  • Practice solving problems involving freezing point depression with different solutes and solvents
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Chemistry students, educators, and professionals involved in physical chemistry or chemical engineering who are interested in understanding colligative properties and their applications in determining molar mass.

AMan24
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Homework Statement


Lauryl alcohol is obtained from coconut oil and is used to make detergents. A solution of 5.00 gram of lauryl alcohol in 0.100Kg of benzene freezes at 4.1 degrees Celsius. What is the molar mass of the lauryl alcohol?
Kf for benzene = 5.12, Freezing point of benzene is 5.5

Homework Equations


Molality = mol of solute/kg solvent
Delta T = (Kf) x (m)

The Attempt at a Solution


(4.1 - 5.5) = (5.12) x (m)
m = -0.27344

-0.27344 = (5.00g/mm)/0.100Kg
5/(-0.27344) x (0.100) = mm

molar mass = -182.855

So when I am doing delta t, its supposed to be final minus initial right? So I am doing the new temperature, 4.1, degrees as the final, and the unchanged or natural temperature, 5.5, as the initial. The answer would be the same thing just positive, but I am probably doing something wrong to have this negative answer. Is delta T supposed to be absolute value? Maybe its supposed to be 5.5 - 4.1, but that doesn't make sense, why would 5.5 be the final temp?

edit: I just found out that Delta T can mean different things. So for this formula, delta T is Tf^0 - Tf. So i figured it out
 
Last edited:
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AMan24 said:
So i figured it out
Way to go.
 

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