Final answer:
The pKa value of the weak acid is calculated using the Henderson-Hasselbalch equation, given the pH of the buffer and the initial molarity of the acid. The number of moles of the acid initially present is determined by multiplying the molarity by the volume of the solution.
Step-by-step explanation:
The question pertains to the calculation of the pKa value of a weak acid and the number of moles of acid initially present when making a buffer solution. To find the pKa, we can employ the Henderson-Hasselbalch equation which relates pH, pKa, and the ratio of the concentrations of the conjugate base (A-) to the weak acid (HA).
Since we are given a pH of 4.07 and we know the initial molarity of the weak acid (0.200 M), we can rearrange the equation to solve for the pKa. Additionally, the number of moles of the weak acid initially present is found by multiplying the molarity of the acid by the volume of the solution in liters (0.715 L).
To provide a concrete example similar to the question asked, let's consider a buffer made with acetic acid (CH3CO2H) and its conjugate base acetate (CH3CO2-). If a buffer solution has a pH slightly changed to 4.75 after adding a strong base, the pKa of the weak acid would be slightly higher than 4.75, since adding a base would increase the concentration of the conjugate base, shifting the pH higher than the pKa. The exact pKa would be calculated using the pH given and the concentrations of the conjugate acid-base pair in the Henderson-Hasselbalch equation. The number of moles initially present would be the initial concentration (0.200 M) times the volume of the solution (0.715 L).