Formic Acid, Which Is a Component of Some Insect Venoms

Multiple Choice

Formic acid, which is a component of some insect venoms, has a K_a = 1.8 × 10^-4. What is the [H₃O⁺] in a solution that is initially 0.10 M formic acid, HCOOH?

A) 4.2 × 10^{-3 M}
B) 8.4 × 10^{-3 M}
C) 1.8 × 10^{-4 M}
D) 1.8 × 10^{-5 M}
E) 1.8 × 10^{-6 M}