Calculating Freezing Point Depression with Urea and Water

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Discussion Overview

The discussion revolves around calculating the amount of urea needed to lower the freezing point of water by 1.60 degrees Celsius. It involves the application of the freezing point depression equation and the concept of molality.

Discussion Character

  • Homework-related

Main Points Raised

  • One participant presents the freezing point depression equation, Tf = Kf x Cm, and provides values for Kf and the desired change in freezing point.
  • Another participant calculates the molality (Cm) using the provided values, resulting in a molality of 0.861.
  • A third participant expresses uncertainty about how to calculate the grams of urea needed based on the molality.
  • A later reply suggests that the mole amount can be determined from the molality and that this value should be multiplied by the molar mass of urea to find the required grams.

Areas of Agreement / Disagreement

The discussion remains unresolved, with participants expressing different levels of understanding and uncertainty regarding the calculation of grams of urea needed.

Contextual Notes

Participants have not explicitly stated all assumptions, such as the volume of the solution or the effects of solute-solvent interactions on freezing point depression.

parwana
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How many grams of urea (MW = 60.056 g/mol) would have to be dissolved in 115.0 grams of water to lower the freezing point by 1.60 degrees celsius?

Equation for freezing point
Tf= Kf x Cm

Kf= 1.858
Cm= molality of solute in kg

please help, iam getting the same answer
 
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Here, [itex]T_f[/itex] is 1.60, and from there you can calculate the molality:

[tex]T_f=K_f \times C_m and C_m=\frac {T_f}{K_f}[/tex]

[tex]C_m=\frac {1.60}{1.858}=0.861[/tex]

Since your molality is 0.861, it means that in 1 kg of solution you'll have to dissolve moles of this amount of compound.
 
i still don't kn ow how to calculate the grams required, please help
 
What did you find as mole amount from the last equation I gave? You'll just multiply this value with molar mass of urea to learn how many grams should be in the solution.
 

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