Question

Part A

Determine the [H3O+] of a 0.230 M solution of formic acid.

Express your answer using two significant figures.

Part B

Determine pH of this solution of formic acid.

Express your answer to two decimal places.

Answer 1

Ans. Create an ICE table as shown in figure.

Part A: Acid dissociation constant for formic acid, Ka = 1.77 x 10^-4

Using ICE table-

                        Ka = [HCOO^-] [H₃O⁺] / [HCOOH]

                        Or, 1.77 x 10^-4 = (X) (X) / (0.230-X)

                        Or, (1.77 x 10^-4) (0.230-X) = X²

                        Or, X² + 0.000177 X - 0.0000407 = 0

Solving the quadratic equation, we get following two roots-

            X1 = 0.00629             ; X2 = - 0.00647

Since concentration can’t be negative, ignore X2.

Therefore, X = 0.00629

Now,

[H₃O⁺] at equilibrium (see, ICE table) = X = 6.3 x 10^-3 M

Part 2: pH of resultant solution = -log [H₃O⁺]

            Or, pH = -log (6.3 x 10^-3) = 2.2

Therefore, pH = 2.2