Freezing point depression constant value clarification.

In summary, the student is struggling to understand why the molar mass of an unknown solute is coming out as a negative value in their freezing point depression lab. They are wondering if they should use the absolute value of the negative Kf value in their calculations in order to get a positive molar mass.
  • #1
LexLuthor
1
0
Hi all,

I just need a little clarification here, please. I'm a tutor and I've been helping a student with their freezing point depression lab and the student has to determine the molar mass of an unknown solute from the experimental data provided. I understand the process and have been able to work all of the equations and explain them to the student without any problems except for one. When the final calculation is made to determine the molar mass of the unknown solute it comes out negative because the Kf provided is negative. That makes sense since it's a freezing point depression constant. However, it does not make sense for a molar mass to be negative. My question is when using the Kf value provided should it just be the absolute value of the Kf? That would make sense because then the molar mass would come out positive as we would expect a molar mass value to be. Any clarification that can be provided will be appreciated. Thanks for taking the time to read this.

- Lex
 
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  • #2
LexLuthor said:
Hi all,

I just need a little clarification here, please. I'm a tutor and I've been helping a student with their freezing point depression lab and the student has to determine the molar mass of an unknown solute from the experimental data provided. I understand the process and have been able to work all of the equations and explain them to the student without any problems except for one. When the final calculation is made to determine the molar mass of the unknown solute it comes out negative because the Kf provided is negative. That makes sense since it's a freezing point depression constant. However, it does not make sense for a molar mass to be negative. My question is when using the Kf value provided should it just be the absolute value of the Kf? That would make sense because then the molar mass would come out positive as we would expect a molar mass value to be. Any clarification that can be provided will be appreciated. Thanks for taking the time to read this.

- Lex

[tex] \Delta T_{f} = K_{f}m [/tex]

[itex] \Delta T_{f} [/itex] and [itex] K_{f} [/itex] are both negative, but the molar mass is still positive, since it is the ratio of two negative numbers.
 

1. What is the freezing point depression constant value?

The freezing point depression constant value, also known as the cryoscopic constant, is a physical constant that represents the amount of change in freezing point for a solution compared to its pure solvent. It is denoted by the symbol "Kf" and has units of degrees Celsius per mole.

2. How is the freezing point depression constant value calculated?

The freezing point depression constant value can be calculated using the equation Kf = ΔTf/m, where ΔTf is the change in freezing point and m is the molality of the solution.

3. What factors affect the freezing point depression constant value?

The freezing point depression constant value is affected by the nature of the solvent, the concentration of solute particles, and the temperature at which the solution is measured. It is also dependent on the type of solute and the strength of its interactions with the solvent molecules.

4. How is the freezing point depression constant value used in scientific research?

The freezing point depression constant value is used in various fields of science, such as chemistry, biochemistry, and environmental science. It is often used to determine the molecular weight of unknown substances, to study the behavior of solutes in different solvents, and to calculate the freezing points of solutions in industrial processes.

5. Can the freezing point depression constant value be affected by external factors?

Yes, the freezing point depression constant value can be affected by external factors such as pressure and impurities in the solution. Changes in pressure can alter the intermolecular interactions between solute and solvent molecules, leading to a change in the freezing point depression constant value. Additionally, impurities in the solution can disrupt the equilibrium between solute and solvent molecules, resulting in a different freezing point depression constant value.

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