A student adds 30.0 mL of 0.200 M HCl to 25.0 mL of 0.100 M NH₃ (aq)
CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases
Problem
A student adds 30.0 mL of 0.200 M HCl to 25.0 mL of 0.100 M NH₃ (aq). Which of the following best explains why the pH of the resulting solution is governed almost entirely by the added HCl rather than by the conjugate acid NH₄⁺ produced in the reaction?
(a) All of the HCl is consumed in the neutralization, leaving only NH4+ in solution; therefore the pH is set by the dissociation of this conjugate acid.
(b) Because the 30.0mL of strong acid is in excess, the remaining HCl supplies a far greater [H+] than the weak conjugate acid NH4+ can generate, so the strong acid controls the pH.
(c) The mixture of the weak base and its conjugate acid forms an effective buffer that holds the pH nearly constant, independent of any excess strong acid that might remain.
(d) The final volume after mixing dilutes any excess HCl to such a low concentration that hydrolysis of NH4+ becomes the chief source of [H+].