Chemistry Practice Problems Ka and Kb Practice Problems Solution: Given a weak acid HA and the following equilibrium...

Solution: Given a weak acid HA and the following equilibrium in water:HA(aq) ⇌ H+(aq) + A–(aq)which answer describes a situation where Kb for the conjugate base A– will have the largest Kb value?A. Ka = 7.0 × 10–8   and   T = 25 °CB. Ka = 7.0 × 10–8   and   T = 80 °CC. Ka = 9.0 × 10–6   and   T = 25 °CD. Ka = 9.0 × 10–6   and   T = 80 °C

Problem

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

HA(aq) ⇌ H+(aq) + A(aq)

which answer describes a situation where Kb for the conjugate base A will have the largest Kb value?

A. Ka = 7.0 × 10–8   and   T = 25 °C

B. Ka = 7.0 × 10–8   and   T = 80 °C

C. Ka = 9.0 × 10–6   and   T = 25 °C

D. Ka = 9.0 × 10–6   and   T = 80 °C

Solution

We are being asked to find determine which situation where Kb for the conjugate base A will have the largest Kb value.

    HA(aq)            H3O+(aq)  +    A-(aq)
     (weak acid)                                        (conjugate base)  


Hence, the Kb of A- can be calculated from the Ka of HA.  Ka and Kb are connected by the autoionization constant of water (Kw) in the following equation:

Kw=Ka·Kb

Kw = 1.0x10-14             @ 25°C

Kw=Ka·KbKwKa=Ka·KbKaKb=KwKa

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