Student Q prepared 50.0 mL of a buffer solution using 0.50 moles of HA and 0.50
ID: 708851 • Letter: S
Question
Student Q prepared 50.0 mL of a buffer solution using 0.50 moles of HA and 0.50 moles of A- while Student S prepared 50.0 mL of a buffer solution using 0.25 moles of HA and 0.25 moles of A-.
a) Do the two buffers solutions have the same or different pH? Explain.
b) If 1.00 mL of 0.010 M NaOH were added to the two buffer solutions, would the pH of the two solutions increase/decrease? Explain.
c) Which solution (Buffer Q or S) would show a smaller change in the pH for question (b)? Explain.
Sorry, I don't know where to start or begin. I've been reading my textbook, yet still don't understand how to solve this problem.
Explanation / Answer
a) pH = pka + log[A-]/[HA],
student Q [HA] =moles/vol in liters = (0.5/0.05) = 10=[A-]
student S [HA] = (0.25/0.05) =5=[A-]
ratio of [A-]/[HA] = 1 for both students sol , hence pH is same for both
b) NaOH dissociates completely to give OH- ions, these OH- reacts with acid to form A- ,
OH- + HA <------> A- + H2O + , hence [HA} decreases while [A] increases,
hence Ratio [A-]/[HA] increases and hence pH increases,
c) Student Q has more moles of HA and A- compared to student S, hence the changes in conc will be relatively smaller compared to student S buffer.
Hence smaller pH change is observed for student Q buffer.