3.2
Titration of a Strong Acid with a Strong Base
During the titration of a strong acid with a strong base, pH calculations are primarily based on the concentration of residual hydronium or hydroxide…
During the titration of a strong acid with a strong base, the pH calculations are based on the concentration of the residual H3O+ or OH− ions.
For instance, 0.1 M HCl dissociates into 0.1 M H3O+ and Cl− ions, starting with an initial pH of approximately 1.
The introduction of 0.1 M NaOH alters the H3O+ concentration in the solution. The new concentration can be calculated by subtracting the total moles of OH− from those of H3O+ and dividing by the total volume. Now, the resultant pH has risen to 1.48.
The continued addition of NaOH solution neutralizes all the H3O+, raising the pH to 7 at the equivalence point.
Beyond this point, OH− govern and determine the pH.
Adding more NaOH solution containing 0.007 moles of OH− results in a final OH− concentration of 0.017 M with the corresponding pOH of 1.77 and pH of 12.23.
Q1: How does pH change when adding a strong base to a strong acid?
When a strong base like sodium hydroxide is added to a strong acid like hydrochloric acid, the hydroxide ions neutralize hydronium ions, reducing their concentration. This causes the pH to increase gradually. The new pH is calculated by determining the concentration of residual hydronium ions remaining after neutralization and dividing by the total solution volume.
Q2: What is the pH at the equivalence point in a strong acid-strong base titration?
At the equivalence point, all hydronium ions from the strong acid have been neutralized by hydroxide ions from the strong base. The resulting solution contains only the salt formed from the reaction, achieving a neutral pH of 7. This occurs when the moles of acid and base are equal.
Q3: How do you calculate pH after the equivalence point is exceeded?
Beyond the equivalence point, excess hydroxide ions govern the pH. Calculate the moles of excess OH− by subtracting the original acid moles from the added base moles. Divide by total volume to find hydroxide concentration, then calculate pOH and convert to pH using the relationship pH + pOH = 14.
Q4: Why does a strong acid have a low initial pH?
A strong acid like hydrochloric acid completely dissociates in water, producing a high concentration of hydronium ions. For example, 0.1 M HCl generates 0.1 M H3O+ ions, resulting in a low pH of approximately 1. The pH is directly related to the high hydronium ion concentration present.
Q5: What happens to the solution composition when a strong base is first added to a strong acid?
When a strong base is initially added, it reacts with hydronium ions present in the acid solution. The hydroxide ions neutralize H3O+ ions, reducing their concentration. The resulting solution contains fewer hydronium ions and more chloride ions, causing the pH to rise above the initial acidic value.
Q6: How is the residual hydronium ion concentration determined during titration?
The residual hydronium concentration is calculated by subtracting the total moles of hydroxide ions added from the original moles of hydronium ions, then dividing by the total solution volume. This gives the concentration of unreacted H3O+ remaining in solution, which is used to calculate the current pH.
Q7: What role do excess hydroxide ions play after the equivalence point?
After the equivalence point, all hydronium ions have been consumed, and excess hydroxide ions remain in solution. These excess OH− ions determine the pH of the solution, making it basic. The concentration of excess hydroxide ions is calculated and used to find pOH, which converts to pH.