Question 1

Multiple choiceMedium1 mark

An aqueous solution of sodium hydrogencarbonate is at 298 K. For H2CO3H_2CO_3 Ka1=4.3×10−7K_{\mathrm a1}=4.3\times10^{-7} and Ka2=4.7×10−11K_{\mathrm a2}=4.7\times10^{-11} Kw=1.0×10−14K_{\mathrm w}=1.0\times10^{-14} Which option pairs the predominant hydrolysis of HCO3−HCO_3^- with the expected pH relative to 7?

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Answer: D. HCO3−HCO_3^- can act as an acid or a base. Its base strength is Kb=Kw/Ka1=2.3×10−8K_{\mathrm b}=K_{\mathrm w}/K_{\mathrm a1}=2.3\times10^{-8} which is greater than its acid strength, Ka2=4.7×10−11K_{\mathrm a2}=4.7\times10^{-11} The predominant hydrolysis produces OH−OH^- so the solution is basic.

A: Treats hydrogencarbonate as an acid and predicts the acidic effect of producing H3O+H_3O^+ although this hydrolysis is weaker.
B: Identifies the predominant hydrolysis but reverses the pH effect of producing OH−OH^-
C: Predicts a basic solution but pairs it with acid hydrolysis, which produces H3O+H_3O^+

Remember: for an amphiprotic ion, compare its acid strength with its base strength.

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