Consider a weak acid, HA. a. Write the equilibrium constant expression for its ionization in aqueous solution. b. Write the expression to describe the degree of ionization (in %).... Consider a weak acid, HA. a. Write the equilibrium constant expression for its ionization in aqueous solution. b. Write the expression to describe the degree of ionization (in %). c. Show the relationships for Ka, Kb and Kw. d. How would you compute for the pH of the solution at equilibrium? e. What type of error is introduced when approximations are made in calculating the pH at equilibrium? f. When are approximations in pH calculations become valid?
Understand the Problem
The question explores the behavior of a weak acid (HA) in an aqueous solution. It asks for the equilibrium constant expression, the degree of ionization, the relationships between Ka, Kb, and Kw, the calculation of pH at equilibrium, the type of error introduced by approximations in pH calculation, and when approximations in pH calculations become valid.
Answer
Here are the answers to your questions: a. Ka = [H+][A-]/[HA] b. Degree of ionization (%) = ([H+]/[HA]initial) * 100 c. Ka * Kb = Kw d. pH = -log[H+] e. Over or Under estimation of pH f. When the acid is very weak, and the concentration is high.
Here are the answers to your questions:
a. Ka = [H+][A-]/[HA] b. Degree of ionization (%) = ([H+]/[HA]initial) * 100 c. Ka * Kb = Kw d. pH = -log[H+], where [H+] is the equilibrium concentration of H+ e. Approximations can lead to either an overestimation or underestimation of the actual pH, depending on the specific conditions and the extent of the approximation. f. Approximations in pH calculations are valid when the acid is very weak, and the concentration is high enough that the change in concentration due to ionization is small compared to the initial concentration.
Answer for screen readers
Here are the answers to your questions:
a. Ka = [H+][A-]/[HA] b. Degree of ionization (%) = ([H+]/[HA]initial) * 100 c. Ka * Kb = Kw d. pH = -log[H+], where [H+] is the equilibrium concentration of H+ e. Approximations can lead to either an overestimation or underestimation of the actual pH, depending on the specific conditions and the extent of the approximation. f. Approximations in pH calculations are valid when the acid is very weak, and the concentration is high enough that the change in concentration due to ionization is small compared to the initial concentration.
More Information
Understanding the conditions under which approximations are valid is crucial for simplifying calculations while maintaining reasonable accuracy.
Tips
A common mistake is applying approximations when they are not valid, leading to significant errors in pH calculations. Always check the validity of the approximation by ensuring that the percent ionization is small (typically less than 5%).
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