Four buffer solutions (W, X, Y, Z) are prepared at using the weak acid HA ( ) and its…

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

Problem

Four 250 mL250\,\text{mL} buffer solutions (W, X, Y, Z) are prepared at 25 ∘C25\,^{\circ}\text{C} using the weak acid HA (Ka=1.0×10−5K_a=1.0\times10^{-5}) and its sodium salt. The component concentrations are listed. W: [HA]=0.40 M[\text{HA}]=0.40\,\text{M}, [A−]=0.10 M[\text{A}^-]=0.10\,\text{M} X: [HA]=0.10 M[\text{HA}]=0.10\,\text{M}, [A−]=0.40 M[\text{A}^-]=0.40\,\text{M} Y: [HA]=0.25 M[\text{HA}]=0.25\,\text{M}, [A−]=0.25 M[\text{A}^-]=0.25\,\text{M} Z: [HA]=0.05 M[\text{HA}]=0.05\,\text{M}, [A−]=0.05 M[\text{A}^-]=0.05\,\text{M} (a) Calculate the initial pH of each buffer. (b) For a single-step addition of 0.015 mol0.015\,\text{mol} of HCl(aq)\text{HCl}(aq) to each buffer (volume change negligible), determine the new pH values and rank the buffers from greatest to least resistance toward the added acid. Support your ranking with quantitative evidence. (c) If instead 0.015 mol0.015\,\text{mol} of NaOH(aq)\text{NaOH}(aq) is added to fresh samples of each buffer, determine the new pH values and produce an analogous ranking for resistance toward base. Explain any differences between the acid-stress ranking and the base-stress ranking.

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