Hydrofluoric acid partially dissociates in water according to at 25 °C In a…
CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
Hydrofluoric acid partially dissociates in water according to
HF(aq)⇌H+(aq)+F−(aq)Ka=6.8×10−4 at 25 °C
In a laboratory experiment a student prepares 250. mL of 0.500 M HF and allows the solution to reach equilibrium at 25 °C. Analytical measurements show that before any further manipulation the equilibrium concentrations are
[HF]=0.486M and [H+]=[F−]=0.014M.
The student then adds 250. mL of pure water to the solution, instantly doubling the volume while maintaining constant temperature.
(1) Immediately after dilution but before any reaction occurs, calculate the concentrations of HF, H+, and F− in the 500. mL solution.
(2) Determine the value of the reaction quotient Qc at this moment and predict the direction in which the system will move to regain equilibrium.
(3) Write algebraic expressions that relate the changes in concentration to a single unknown quantity, then compute the equilibrium concentrations of all three species after dilution.
(4) Using your equilibrium concentrations, calculate the pH of the final solution to two decimal places.