Hydrofluoric acid, HF, has a Ka of 6.8 × 10−4. What are [H3O+], [F−], and [OH−] in 0.820 M HF?

Question

Hydrofluoric acid, HF, has a K_a of 6.8 × 10⁻⁴. What are [H₃O⁺], [F⁻], and [OH⁻] in 0.820 M HF?

please help me fill in the blanks

[H₃O⁺] = ____ M

[F⁻] = ____ M

[OH⁻] = ____ x 10^ __ M. (Enter your answer in scientific notation for this one.)

Answer 1

Easiest to set up an ICE table, but not essential to do so.

HF + H₂O ===> H₃O⁺ + F^-

0.820.....................0............0.......Initial

-x........................+x...........+x.......Change

0.820-x...............x..............x........Equilibrium

Ka = 6.8x10^-4 = [H₃O⁺][F^-]/[HF]

6.8x10^-4 = (x)(x) / 0.820 - x

x² = 5.6x10^-4 - 6.8x10^-4x

x² + 6.8x10^-4x - 5.6x10^-4 = 0

x = 0.0233 M = [H₃O⁺] = [F^-]

[H3O⁺][OH^-] = 1x10^-14

[OH-] = 1x10^-14/[H₃O⁺] = 1x10^-14 / 2.33x10^-2

[OH-] = 4.3x10^-13 M