Calculate the pH when the following volumes of 10^(-1,1) M XOH are added into 1 L of a 10^(-2.5) M HA solution. XOH is a strong base and HA is a weak monoprotic acid (pKa=4.5)
To calculate the pH when a strong base is added to a weak acid solution, we need to follow the steps for calculating the pH of a weak acid solution and then consider the effect of adding a strong base.
Step 1: Calculate the initial concentration of the weak acid (HA)
Given that the initial concentration of HA is 10^(-2.5) M, the initial concentration of HA can be represented as [HA]₀ = 10^(-2.5) M.
Step 2: Determine the equilibrium concentration of H₃O⁺ ions
For a weak monoprotic acid like HA, we can assume that the dissociation of HA is small compared to its initial concentration. Therefore, we can assume that its concentration at equilibrium ([HA]) is approximately equal to its initial concentration ([HA]₀).
Step 3: Calculate the equilibrium concentration of the conjugate base (A⁻)
Since HA is a weak acid, it will partially dissociate into its conjugate base A⁻. The equilibrium concentration of A⁻ can be calculated using the equation:
[A⁻] = [OH⁻]
Step 4: Calculate the concentration of OH⁻ ions
Given that the volume of the 10^(-1,1) M XOH added to the solution is known, we can calculate the amount of XOH added using the formula:
Amount of XOH = Volume × Concentration
Here, Volume is the volume of the 10^(-1,1) M XOH, and Concentration is 10^(-1,1) M.
Step 5: Determine the change in concentration of HA and A⁻
Since HA reacts with OH⁻ to form A⁻, the change in concentration of HA and A⁻ is given by:
Change in [HA] = -[OH⁻]
Change in [A⁻] = [OH⁻]
Step 6: Calculate the equilibrium concentrations of HA and A⁻
To calculate the equilibrium concentration of HA and A⁻, we need to use the initial concentrations and the changes in concentration calculated in the previous steps:
[HA] = [HA]₀ + Change in [HA]
[A⁻] = Change in [A⁻]
Step 7: Calculate the pH
Finally, we can use the Henderson-Hasselbalch equation to calculate the pH of the solution:
pH = pKa + log([A⁻]/[HA])
Substituting the values for [A⁻] and [HA] into the equation will give you the pH of the solution after the addition of the XOH solution.