Question

calculate the pH of a 1M weak acid solution for which pKa=6

Answer 1

Setting up an iCE table of weak acid HA with fractional dissociation x, we get:

                                            HA    $\rightarrow$          H⁺     +      A^-

Initial concentration:              1                   0               0

Equilibrium concentration:   1 - x                x                x

We have the relation: pK_a = -log(K_a)

K_a = 10^-pKa = 10^-6

Writing the equation for K_a, K_a = [H⁺} * [A^-] / [HA]

Using the values from the table: 10^-6 = x * x / (1-x)

Solving, x = [H⁺] = 10^-3

pH = -log( [H⁺]) = -log(10^-3) = 3