An analytical laboratory prepares of hydrochloric acid that has a nominal…
CHEMISTRY · Multi-part · AP · Chemistry · Acids and Bases · Acids and Bases
Problem
An analytical laboratory prepares 50.0mL of hydrochloric acid that has a nominal concentration of 1.0×10−7M at 25∘C. This solution will serve as the analyte in a titration; however, the pH electrode is first used to measure the initial pH before any titrant is delivered.
(a) Explain why taking the negative logarithm of the nominal HCl concentration would not yield an accurate pH for this solution.
(b) Derive an expression for the total hydronium-ion concentration, [H3O+]total, that accounts for both the dissociation of the acid and the auto-ionization of water.
(c) Solve the expression obtained in part (b) to determine the numerical value of [H3O+]total and the corresponding pH to two decimal places.
(d) Discuss how the magnitude of the error made by using the nominal concentration alone changes as the acid concentration becomes even smaller (e.g., 1.0×10−8M).