Three phosphate buffers are prepared at 25 °C using the conjugate pair ( )

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

Problem

Three phosphate buffers are prepared at 25 °C using the

\mathrm{H_2PO_4^- / HPO_4^{2-}}

conjugate pair (

pK_a = 7.21

). • Buffer E: 0.20 M

\mathrm{H_2PO_4^-}

and 0.20 M

\mathrm{HPO_4^{2-}}

(total volume 150 mL) • Buffer F: 0.20 M

\mathrm{H_2PO_4^-}

and 0.05 M

\mathrm{HPO_4^{2-}}

(total volume 150 mL) • Buffer G: 0.80 M

\mathrm{H_2PO_4^-}

and 0.80 M

\mathrm{HPO_4^{2-}}

(total volume 150 mL) Answer the questions below regarding their capacity to withstand additions of strong acid and strong base. (a) Rank the three buffers in order of their capacity to neutralize added strong acid. Justify your ranking quantitatively or qualitatively. (b) Rank the three buffers in order of their capacity to neutralize added strong base. Explain any differences from the ordering in part (a). (c) For Buffer E, calculate the maximum number of moles of 6.0 M

\mathrm{HCl}

that can be added without changing the pH by more than 0.50 unit. (d) Suppose an equal number of moles of 6.0 M

\mathrm{HCl}

is added to each buffer. Predict which buffer will experience the smallest pH change and support your prediction with numerical reasoning.

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