MeOH:H2O Freezing Point Depression Calculation Issue

In summary, the conversation discusses the use of methanol in windshield washer fluid to reduce its freezing temperature. The calculation of the required amount of methanol is done using the mSolute = dT/Kf formula, which gives a result of 15.054 mol/kg. However, this formula is invalid for methanol as it is a volatile solute. This means that the actual concentration of methanol needed may be different.
  • #1
Voltux
30
3
I noticed that my windshield washer fluid has methanol in it to reduce the temperature of which it freezes. I know it freezes at approximately -28°C which gives us a concentration of MeOH at approximately ~30% by weight.

However, when I try to calculate the required methanol using the mSolute = dT/Kf I get 15.054 mol/kg which is 482.5g/L of MeOH. That turns out to be about 609mL/L if you convert that to volume with the density of 0.792g/mL.

Can someone explain where I went wrong in my calculations, or how to properly calculate this?

Data:
Kf Constant for Water: 1.86
MeOH Molar Mass: 32.05g/mol
MeOH Density: 0.792g/mL
Temperature Difference: 28°
 
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  • #2
First of all, you are using an invalid model.

Freezing point depression with Kf assumes the solute to be nonvolatile, that's definitely not a case with the methanol.
 

1. What is MeOH:H2O freezing point depression?

MeOH:H2O freezing point depression is a phenomenon that occurs when a solvent mixture of methanol and water is cooled below its freezing point. This results in a lowering of the freezing point of the solution compared to the pure components.

2. How is MeOH:H2O freezing point depression calculated?

MeOH:H2O freezing point depression can be calculated using the equation ΔT = Kf * m * i, where ΔT is the change in freezing point, Kf is the cryoscopic constant, m is the molality of the solution, and i is the van't Hoff factor which takes into account the dissociation of the solute.

3. What causes MeOH:H2O freezing point depression?

MeOH:H2O freezing point depression is caused by the presence of a solute, in this case methanol, in a solvent, water. The solute molecules disrupt the orderly arrangement of solvent molecules, making it more difficult for the solvent to freeze.

4. How does MeOH:H2O freezing point depression affect experiments?

MeOH:H2O freezing point depression can affect experiments by altering the freezing point of the solution, which may impact the accuracy of measurements or the formation of crystals in the solution. It is important to account for this phenomenon when conducting experiments with solvent mixtures.

5. Is MeOH:H2O freezing point depression the same as boiling point elevation?

No, MeOH:H2O freezing point depression and boiling point elevation are two different phenomena. While freezing point depression occurs when a solute is added to a solvent, boiling point elevation occurs when a non-volatile solute is added to a solvent, resulting in an increase in the boiling point of the solution.

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