If the Kₐ of a monoprotic weak acid is 3.9 × 10⁻⁶, what is the [H⁺] of a 0.50 M solution of this acid?
a) 3.9 × 10⁻³ M
b) 7.8 × 10⁻⁴ M
c) 1.95 × 10⁻³ M
d) 1.3 × 10⁻⁵ M