What is the definition of K_b? Write out the equalibrium reaction of a generic b

Question

What is the definition of K_b? Write out the equalibrium reaction of a generic base, B, that defines the equilibrium constant K_b. Write the general expression you would use to calculate K_b from concentration(s). How are K_b and pK_b related? How would you calculate one from the other? How does the K_b value of a base relate to the pH of its aqueous solution? How would you change the reaction you wrote in part (a) if you were instead asked about K_b for the conjugate base of the acid HA? When and how are the K_a of an acid and the K_b of a base related?

Explanation / Answer

a) a constant that depends upon the equilibrium between the ions and the molecules that are not ionised in a solution or liquid called as dissociation constant

B + H2O ---------------> BH+   + OH-

Kb = [BH+][OH-]/[B]

b)

if Kb is given pKb can be calculated : pKb = -log Kb

if pKb is given Kb can be calculated : Kb = 10^-pKb

c )

Kb increases basic strength increase pH increases

pH =14 - 1/2 [pKb -logC]

d)

A-   + H2O ------------------> HA + OH-

Kb = [HA][OH-]/[A-]

e)

for acid Ka value for conjugate base Kb value used

Ka x Kb = Kw

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