CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A weak base B has a base-dissociation constant Kb=3.5×10−3 at 25∘C. A student prepares 250.0mL of an aqueous solution by dissolving 5.0×10−5mol of B.
(a) Calculate the pOH and pH of the solution assuming that the equilibrium concentration of B is essentially equal to its initial concentration.
(b) Determine the exact pOH and pH of the solution by solving for all equilibrium concentrations without using the assumption in part (a).
(c) Compute the percent difference between the [OH−] values obtained in parts (a) and (b).
(d) Discuss whether the approximation used in part (a) is acceptable for this particular solution, citing quantitative evidence from your calculations.