16.6
Titration Calculations: Strong Acid - Strong Base
Calculating pH for Titration Solutions: Strong Acid/Strong Base
A titration is carried out for 25.00 mL of 0.100 M HCl (strong acid) with 0.100 M of a…
When a strong acid is titrated with a strong base or vice versa, the pH throughout the titration can be calculated by determining the concentration of the remaining hydronium or hydroxide ions.
For example, 50 mL of 0.10 M hydrochloric acid dissociates into 0.10 M hydronium and chloride ions with an initial pH of one.
If 25 mL of 0.10 M sodium hydroxide is added, the new concentration of hydronium ions can be calculated by subtracting the total moles of hydroxide ions from the total moles of hydronium ions and dividing it by the total volume—75 mL or 0.075 L.
50 mL of 0.10 M hydrochloric acid contains 0.0050 moles of hydronium, while 25 mL of 0.10 M sodium hydroxide has 0.0025 moles of hydroxide ions. Substituting these values into the equation, the hydronium ion concentration equals 0.033 M. Therefore, the pH of the solution has risen to 1.48.
If the addition of sodium hydroxide is continued up to 50 mL, all the hydronium ions from hydrochloric acid get neutralized, and the equivalence point is reached when the pH rises to seven.
Above the equivalence point, the pH of the solution is determined by the hydroxide ions, as the hydrochloric acid has been completely neutralized.
If 70 mL of sodium hydroxide solution containing 0.0070 moles of hydroxide ions is added into the solution, the final concentration of hydroxide ions can be determined by subtracting the initial moles of hydronium ions from the total moles of hydroxide ions and dividing it by the total volume of the solution.
As the final hydroxide ion concentration is 0.017 M, the pOH and pH of the solution equal 1.78 and 12.22, respectively.
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Q1: How do you calculate pH before any base is added to a strong acid?
For a strong acid like hydrochloric acid, complete ionization occurs, so the hydronium ion concentration equals the acid's molarity. For example, 0.10 M HCl produces 0.10 M hydronium ions, giving a pH of 1.0. Simply use pH = -log[H+] to find the initial pH of the solution.
Q2: What happens to pH when you add base before reaching the equivalence point?
Before the equivalence point, excess acid remains in solution. Calculate the moles of hydronium ions remaining by subtracting moles of added hydroxide from initial moles of hydronium, then divide by total volume. For instance, adding 25 mL of 0.10 M NaOH to 50 mL of 0.10 M HCl leaves 0.0025 moles of hydronium in 75 mL, yielding pH 1.48.
Q3: What is the pH at the equivalence point for a strong acid-strong base titration?
At the equivalence point, all hydronium ions from the acid are neutralized by hydroxide ions from the base, producing only water and salt. Neither the cation nor anion undergoes acid-base ionization, so only water autoprotolysis generates hydronium ions. The pH equals 7.0, making the solution neutral.
Q4: How do you determine pH after adding excess base past the equivalence point?
Beyond the equivalence point, all acid is consumed and pH is determined by excess hydroxide ions. Calculate the moles of hydroxide remaining by subtracting initial moles of hydronium from total moles of hydroxide, then divide by total volume. Use pOH = -log[OH-] and pH = 14 - pOH to find the final pH.
Q5: Why does pH change more dramatically near the equivalence point in strong acid-strong base titrations?
Near the equivalence point, small additions of base cause large changes in hydronium ion concentration because the buffer capacity is minimal. Before the equivalence point, excess acid resists pH change, but as the equivalence point approaches, nearly all acid is consumed, making the solution sensitive to small base additions.
Q6: What is the relationship between moles of acid and moles of base at the equivalence point?
At the equivalence point, the moles of hydronium ions from the acid equal the moles of hydroxide ions from the base. For monoprotic strong acid and strong base, this means the moles of acid and base are equal. Using the equation moles = molarity × volume, you can determine the volume of titrant needed to reach equivalence.
Q7: How does the volume of titrant added affect the total volume used in pH calculations?
The total volume in pH calculations includes both the initial acid solution and all added base titrant. For example, adding 25 mL of base to 50 mL of acid gives a total volume of 75 mL. This dilution effect is critical because it reduces the concentration of remaining hydronium or excess hydroxide ions, directly affecting the calculated pH.