acid base equilibrium

In the solution, water undergoes self-ionization to give H+(aq) and OH- ions.
H2O(l) ≒ H+(aq) + OH-(aq)
Kw = [H+][OH-] = 1 x 10-14 M2

Since Kw is too small, the concentration of H+(aq) ions depends almost entirely on the dissociation of HA(aq). However, OH-(aq) ions also exist in the solution due to the self-ionization of water.

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Consider the dissociation of HA:
startaa aaaaHA(aq) ≒ A-(aq) + H+(aq)
start aaaa 0.01 Mq) + HA0 Mq) + H0 M
change aaa -y M a(l) ≒ +y M) +H+y M
eqm aaaa(0.01 - y) M+HAy M) + H3y M

Ka = y2 / (0.01 - y) = 1.7 x 10-10 (M)
y2 + (1.7 x 10-10)y - (1.7 x 10-12) = 0
[H+] = y = 1.3 x 10-6 M

Consider the self-ionization of water:
eqm H2O(l) ≒ OH-(aq) + H+(aq)
eqm 2H2O(l)≒ H? M q 1.3 x 10-6 M

Kw = [OH-][H+]
[OH-] = Kw/[H+] = (1 x 10-14)/(1.3 x 10-6) = 7.69 x 10-9 M