Question #7213b

1 Answer
Al E.
Jan 21, 2018

Consider the weak acids equilibrium

#HA(aq) rightleftharpoons H^(+)(aq) + A^(-)(aq)#, where

#K_a = ([H^+][A^(-)])/([HA]) = 10^-4.30 approx 5.01*10^-5#

And concomitantly,

#K_b = (K_w)/K_a = 2.00*10^-10#

Now, consider the basic salt equilibrating,

#A^(-)(aq) + H_2O(l) rightleftharpoons HA(aq) + OH^(-)(aq)#, where

#K_b = ([HA][OH^-])/([A^-]) = 2.00*10^-10#

Now, assuming that #[HA] = [OH^-]# due to hydrolysis for ease of calculation,

#2.00*10^-10 = (2.00*10^-6)^2/x#

#therefore x = [A^-] = [NaA] approx 0.02M#

Common mistakes in these problems include incorrect equilibrium constant use, and wrong representations of equilibrium expressions (which I made!). I'm open to feedback if I made a mistake!

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