Two 100.0 mL samples of aqueous ammonia buffer are prepared at 25 °C

CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases

Problem

Two 100.0 mL samples of aqueous ammonia buffer are prepared at 25 °C. The acid–base pair is

\mathrm{NH_4^+/NH_3}

with

pK_a = 9.25

. • Buffer X contains 0.25 M

\mathrm{NH_4Cl}

and 0.25 M

\mathrm{NH_3}

. • Buffer Y contains 0.050 M

\mathrm{NH_4Cl}

and 0.050 M

\mathrm{NH_3}

. Hydrochloric acid (

\mathrm{HCl}

, 1.00 M) will be added dropwise to each buffer. (a) Calculate the initial pH of Buffer X and of Buffer Y. (b) Determine the number of moles of

\mathrm{HCl}

that must be added to Buffer X to lower its pH by exactly 0.20 unit. (c) Determine the number of moles of

\mathrm{HCl}

that must be added to Buffer Y to produce the same 0.20-unit pH decrease. (d) Identify which buffer has the higher acid-neutralizing capacity and by what numerical factor, based on your calculations.

Related theory

Equilibrium
Buffer Solutions

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