A student titrates 0.025 M KOH into 50.00 ml of a solution of unknown weak acid HA. Equivalence point is reached when 30.00 ml of the KOH has been added. After 15.00ml of the KOH has been added the pH of the mixture is 3.92.
A. What is the concentration of HA ?
B. What is Ka for HA ?
C. What is the pH of the HA before any KOH is added?
D. What is the pH at equivalence point?
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To find the answers to these questions, we need to use the principles of acid-base titrations and the calculations associated with them.
A. To find the concentration of HA, we need to use the stoichiometry of the reaction between KOH and HA. At the equivalence point, the moles of KOH added are equal to the moles of HA present in the initial solution.
Given:
- Volume of KOH solution added at the equivalence point = 30.00 ml = 0.030 L
- Concentration of KOH = 0.025 M
Using the equation:
Moles of KOH = concentration of KOH × volume of KOH solution
Moles of HA = Moles of KOH
Therefore, the concentration of HA can be calculated as follows:
Concentration of HA = Moles of HA / Volume of solution
B. To find the Ka for HA, we need to use the pH value at a certain point during the titration. The pH at the midpoint of a weak acid-strong base titration is equal to the pKa of the acid. The pKa can then be converted to Ka using the formula Ka = 10^(-pKa).
Given:
- pH after adding 15.00 ml of KOH = 3.92
C. To find the initial pH of HA before any KOH is added, we need to consider the dissociation of HA. HA is a weak acid, which means it partially dissociates into its conjugate base and hydrogen ions (H+).
D. To find the pH at the equivalence point, we need to understand the nature of the reaction at that point. At the equivalence point, all the weak acid has reacted with the strong base, resulting in a solution containing only the salt of the weak acid. The pH can be calculated by considering the dissociation of the salt and the hydrolysis reaction.
Let's now calculate the answers using the given information.