CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Methylamine, CH3NH2 (Kb=4.4×10−4), is titrated with 0.200 M sulfuric acid at 25 °C. For the purpose of this problem, assume that only the first proton of H2SO4 reacts quantitatively during the titration and the second proton remains unreactive in the volume range considered.
A 40.00 mL sample of 0.100 M CH3NH2 is titrated, and pH is recorded when 12.0 mL of the acid has been delivered.
(a) Write the balanced net ionic equation for the reaction between methylamine and the first acidic proton of sulfuric acid.
(b) Determine the number of moles of each reactant present before reaction at 12.0 mL of added acid, and identify the species in excess after the reaction.
(c) Calculate the molarity of excess hydrogen ions in the final solution volume.
(d) Compute the pH of the solution at this point in the titration.
(e) If the titration were repeated with 0.200 M hydrochloric acid instead of sulfuric acid, explain qualitatively whether the pH at 12.0 mL of acid added would be the same, higher, or lower. Justify your reasoning without performing additional numerical calculations.