A sample of 0.314g of unknown monoprotic weak acid, HA was dissolved in 25.00 mL of water and titrated with 0.1M KOH. The acid required 27.20 mL of the base to reach equilibrium point.
Find the pH of the solution if 40 mL KOH is added to the acid solution.
My question is how I determine the acid dissociation constant with these information in order to find pH of solution.
HA + KOH ==> KA + H2O
If the acid requires 27.20 mL of 0.1M KOH to reach the equivalence, then 40 mL of the 0.1M KOH will neitralize all of the acid and have excess KOH reamaining. Therefore, the excess KOH will determine the pH of the solution and the Ka for the HA is not needed.
Thank you very much.
To determine the acid dissociation constant (Ka), you can use the information provided in the question along with the balanced chemical equation for the acid-base reaction:
HA + OH- -> A- + H2O
Given:
Mass of HA = 0.314g
Volume of solution = 25.00 mL = 0.02500 L
Volume of KOH required for neutralization = 27.20 mL = 0.02720 L
Concentration of KOH = 0.1 M
Step 1: Calculate the number of moles of KOH used:
Moles of KOH = concentration × volume = 0.1 M × 0.02720 L = 0.00272 mol
Step 2: Convert the number of moles of KOH used to moles of HA:
From the balanced equation, 1 mol of KOH reacts with 1 mol of HA, so moles of HA = 0.00272 mol
Step 3: Calculate the molar mass of HA:
Molar mass of HA = mass / moles = 0.314 g / 0.00272 mol = 115.44 g/mol
Step 4: Calculate the initial concentration of HA:
Initial concentration of HA = mass / volume = 0.314 g / 0.02500 L = 12.56 g/L
Step 5: Calculate the final concentration of HA after adding the additional 40 mL of KOH:
Volume of solution after adding KOH = 0.02500 L + 0.04000 L = 0.06500 L
Final concentration of HA = mass / volume = 0.314 g / 0.06500 L = 4.83 g/L
Step 6: Calculate the concentration of A- after neutralization:
Neutralization of HA with KOH results in the complete conversion of HA to A-. Therefore, the concentration of A- is equal to the final concentration of HA:
Concentration of A- = 4.83 g/L
Step 7: Calculate the concentration of OH- after neutralization:
Using the stoichiometry of the reaction, the moles of OH- used for neutralization will be the same as the moles of A- produced. Therefore, the concentration of OH- is equal to the concentration of A-:
Concentration of OH- = concentration of A- = 4.83 g/L
Step 8: Calculate the concentration of H+ (protons) in the solution:
Since water is a neutral compound, the concentration of H+ is equal to the concentration of OH-. Therefore, the concentration of H+ or pOH can be calculated using the equation:
[H+] = Kw / [OH-]
where Kw is the ion product of water (1 x 10^-14).
Step 9: Calculate pOH:
pOH = -log([OH-])
Step 10: Convert pOH to pH:
pH = 14 - pOH
Now, using these steps, you can determine the acid dissociation constant (Ka) to find the pH of the solution.