We are being asked to find determine which situation where K_{b} for the conjugate base A^{–} will have the largest K_{b} value.

** HA(aq) ****⇌**** H_{3}O^{+}(aq) + A^{-}(aq)**

Hence, the K_{b} of A^{-} can be calculated from the K_{a} of HA. **K _{a} and K_{b} **are connected by the

$\overline{){{\mathbf{K}}}_{{\mathbf{w}}}{\mathbf{=}}{{\mathbf{K}}}_{{\mathbf{a}}}{\mathbf{\xb7}}{{\mathbf{K}}}_{{\mathbf{b}}}}$

**K _{w} = 1.0x10^{-14} @ 25**

${\mathbf{K}}_{{\mathbf{w}}}\mathbf{=}{\mathbf{K}}_{{\mathbf{a}}}\mathbf{\xb7}{\mathbf{K}}_{{\mathbf{b}}}\phantom{\rule{0ex}{0ex}}\frac{{\mathbf{K}}_{\mathbf{w}}}{{\mathbf{K}}_{\mathbf{a}}}\mathbf{=}\frac{\overline{){\mathbf{K}}_{\mathbf{a}}}\mathbf{\xb7}{\mathbf{K}}_{\mathbf{b}}}{\overline{){\mathbf{K}}_{\mathbf{a}}}}\phantom{\rule{0ex}{0ex}}\overline{){{\mathbf{K}}}_{{\mathbf{b}}}{\mathbf{=}}\frac{{\mathbf{K}}_{\mathbf{w}}}{{\mathbf{K}}_{\mathbf{a}}}}$

Given a weak acid HA and the following equilibrium in water:

HA(aq) ⇌ H^{+}(aq) + A^{–}(aq)

which answer describes a situation where K_{b} for the conjugate base A^{–} will have the largest K_{b} value?

A. K_{a} = 7.0 × 10^{–8} and T = 25 °C

B. K_{a} = 7.0 × 10^{–8} and T = 80 °C

C. K_{a} = 9.0 × 10^{–6} and T = 25 °C

D. K_{a} = 9.0 × 10^{–6} and T = 80 °C

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