Question

Formic acid (HCHO2) is a component of bee stings. It is a weak acid and dissociates according to the
equilibrium shown below:

HCHO2 <===> H+ + CHO2- Kc = 1.8 x 10-4

For a 0.5 M solution of formic acid, we wish to determine the equilibrium concentration of H+.

Complete the ICE table below. Show your calculation for the reaction quotient, Qc, and use that to decide on the signs (+ or -) for the changes in the reactant and products. Briefly explain why the signs for the changes in the reactant and products are the way they are.

Determine the equilibrium concentration of H+. (Hint: is the equilibrium constant small enough to use
the "x is small" approximation?)

[HCHO2](M) [H+](M) [CHO2-](M)
Initial 0.5 0 0

Change

Equilibrium

Answer 1

Initial

[HCHO2](M)= 0.5
[H+](M) = 0
[CHO2-](M)= 0

Change = ?

Equilibrium = ?

Answer 2

..............HCHO2 ==> H^+ + CHO2^-
initial.......0.5.......0......0
change.........-x........x......x
equil........0.5-x.......x......x

Substitute the ICE chart values into K
Ka = (H^+)(CHO2^-)/(HCHO2) and solve for (H^+) (which in the ICE chart is x).