Question

I need to calculate the vapor pressure of an acid in equilibrium with an aqueous solution of its salt. For example, say I have a solution of 40% potassium formate by mass in water at 50°C. How do I find the vapor pressure of formic acid in equilibrium with that liquid?

My best guess is that I first calculate the concentration of formate ions via pKb = -log10([HB+][OH-]/[B]).

Then I can calculate the concentration of formic acid using pKa=log10([HA]/[A-][H+]).

Then I can use Raoult's law to estimate the partial pressure.

This is the best I can come up with, but it doesn't feel quite right to me. Does anyone recommend any corrections to this process?

Answer 1

To estimate the vapor pressure of formic acid in equilibrium with its salt, write a balanced chemical equation for its dissociation, calculate the solution's pH using the acid dissociation constant and Henderson-Hasselbalch equation, determine the acid concentration, and apply Raoult's law.

Step-by-step explanation:

To calculate the vapor pressure of formic acid in equilibrium with an aqueous solution of its salt, such as a solution of 40% potassium formate by mass in water at 50°C, the student's initial approach requires some adjustments. Instead of using the base ionization constant (pKb), we should directly use the acid dissociation constant (Ka) of formic acid because we are dealing with the acid itself and its conjugate base in solution. The vapor pressure of formic acid can be calculated by applying the principles of chemical equilibrium and Raoult's law.

Strategy:

1. Write a balanced chemical equation for the dissociation of formic acid in water.
2. Calculate the concentration of the formate ions from the salt in solution.
3. Use the Henderson-Hasselbalch equation to find the pH of the solution.
4. Using the pH, calculate the concentration of formic acid [HCO₂H].
5. Apply Raoult's law to estimate the vapor pressure of formic acid in the solution.

This process will provide an approximation for the vapor pressure of formic acid in the solution described. Keep in mind that the volatile nature of formic acid may not make Raoult's law an ideal model if the acid is present at significant concentrations or if non-ideal behavior is considerable.