A buffer is prepared by mixing equal concentrations of and ions

CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases

Problem

A buffer is prepared by mixing equal concentrations of H2PO4−\mathrm{H_2PO_4^-} and HPO42−\mathrm{HPO_4^{2-}} ions. I. A small amount of HCl(aq) is added. II. A small amount of NaOH(aq) is later added to the same solution. Which pair of net ionic equations correctly represents the predominant reactions in events I and II, respectively? (a) I H++H2PO4−→H3PO4\mathrm{H^+ + H_2PO_4^- \rightarrow H_3PO_4} II OH−+HPO42−→PO43−+H2O\mathrm{OH^- + HPO_4^{2-} \rightarrow PO_4^{3-} + H_2O} (b) I H++HPO42−→H2PO4−\mathrm{H^+ + HPO_4^{2-} \rightarrow H_2PO_4^-} II OH−+H2PO4−→HPO42−+H2O\mathrm{OH^- + H_2PO_4^- \rightarrow HPO_4^{2-} + H_2O} (c) I H++HPO42−→H2PO4−\mathrm{H^+ + HPO_4^{2-} \rightarrow H_2PO_4^-} II OH−+HPO42−→PO43−+H2O\mathrm{OH^- + HPO_4^{2-} \rightarrow PO_4^{3-} + H_2O} (d) I H++H2PO4−→H3PO4\mathrm{H^+ + H_2PO_4^- \rightarrow H_3PO_4} II OH−+H2PO4−→HPO42−\mathrm{OH^- + H_2PO_4^- \rightarrow HPO_4^{2-}}

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