Final answer:
In a titration of a weak acid with a strong base, the pH at the halfway point is equal to the pKa of the weak acid and the pH is approximately 8.72.
Step-by-step explanation:
In a titration of a weak acid with a strong base, the halfway point is known as the equivalence point. At this point, half of the weak acid has reacted with the base. The pH at the equivalence point is equal to the pKa of the weak acid. If the pKa is known, the pH can be calculated.
The excess hydroxide ions from the base will make the solution basic, so we can use stoichiometric calculations to determine the pH. For example, if 37.50 mL of a 0.100 M NaOH solution is added and the pKa is 5.23, the pH can be calculated as follows:
First, calculate the moles of NaOH added: 0.03750 L x 0.100 M = 0.003750 mol NaOH.
Since this is past the equivalence point, we have excess hydroxide ions. The concentration of hydroxide ions is equal to the moles of NaOH divided by the final volume of the solution. From the given information, the final volume of the solution is not provided, so we cannot calculate the concentration of hydroxide ions precisely. However, we can estimate the pH range. If the moles of hydroxide ions divided by the final volume of the solution is approximately 5.3 x 10^-6 M, the pOH can be calculated as -log(5.3 x 10^-6) = 5.28. Finally, the pH can be calculated as 14.00 - 5.28 = 8.72.
Therefore, at the halfway point when 1/2 of NH3 has reacted, the pH is approximately 8.72.