Suppose an equilibrated, dilute solution containing an acid HA with Ka = 10⁻4 is measured to have pH = 6 and [HA] = 10⁻8 M. Estimate [A⁻].
a) 10⁻² M
b) 10⁻⁴ M
c) 10⁻⁶ M
d) 10⁻⁸ M