 # Freezing Point Depression for Strong and Weak Electrolytes

## Transcribed Text

Determination of the Freezing Point Depression for Strong and Weak Electrolytes 1. Complete the following table, summarizing the results of the experiment. The molality is the value determined from the titration with NaOH. Solution Average Tf (°C) Standard Deviation of Tf Average molality (m) Mol/kg Standard Deviation of molality 0.25 m CH2ClCOOH 0.125 m CH2ClCOOH 0.25 m HCl 0.125 m HCl 2. Calculate the Ka and for each solution of chloroacetic acid using the data from the table above. The value for m’ can be calculated from Equation 6. Ka and can be determined from Equation 9. Summarized the data in the table below and append sample calculations. Solution m’ (mol/kg) Ka 0.25 m CH2ClCOOH 0.125 m CH2ClCOOH 3. What is the literature value for the Ka at 0 °C? Compare this value to the value obtained in your experiment for both the 0.25 m and 0.125 m solutions of chloroacetic acid. Are the values the statistically the same? 4. Complete the table below. Calculate the experimental value of gl for each solution of hydrochloric acid using Equation 10. Calculate the theoretical value of gl using Equation 13. Append your calculations. Solution gl(experimental) gl (theoretical) 0.25 m HCl 0.125 m HCl 5. The osmotic coefficient is a measure of a solution’s deviation from ideality. How does the value of gl change with concentration? What does that tell you? 6. Discuss potential sources of error in the experiment. 7. For a solution of HA with a molality of m, write expressions (in terms of m and the fraction dissociated, ) for the concentrations of HA, H+ and A- in a solution. [Hint: start with an ICE chart] 8. Using your answer from number 6, show how the total concentration of species in solution is: m' m() 9. In General Chemistry, you were presented with the equation: iKf m where i is called the Van’t Hoff factor. The Van’t Hoff factor is a measure of the effective molality of a solution. In other words, a solute such as HCl will not produce 2 moles of ions for each mole of the acid, which would be expected for an ideal solution. This can be expressed as: i mapp /m where mapp is the apparent molality of a solution. What is the relationship between the Van’t Hoff factor, i, and the osmotic coefficient, gl? Show this relationship with an equation.

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