What is the approximate pH at the equivalence point of a weak acid-strong base titration if 25 mL of aqueous formic acid requires 29.80 mL of 0.2567 M NaOH? K_a = 1.8 × 10^-4 for formic acid. A)2.13 B)5.56 C)8.44 D)11.87

Question

Quizplus · Accepted Answer

At the equivalence point of a weak acid-strong base titration, the solution will have a pH greater than 7 because the strong base will neutralize all of the weak acid and the remaining solution will consist of the conjugate base. To determine the pH, we need to calculate the moles of formic acid present in the 25 mL sample.
moles of HCOOH = Molarity x Volume = 0.2567 M x 0.02980 L = 0.007642 mol
According to the balanced equation, the reaction between formic acid and sodium hydroxide is:
HCOOH + NaOH -> NaCOOH + H2O
1 mole of HCOOH reacts with 1 mole of NaOH
So, at the equivalence point, 0.007642 moles of NaOH will have been added to the solution. This means that 0.007642 moles of HCOOH have been neutralized and converted to NaCOOH. The remaining concentration of HCOOH will be:
[HCOOH] = [HCOOH]initial - [NaOH]added = 0.1 M - (0.2567 M x 0.02980 L/0.02500 L) = 0.0667 M
Using the equilibrium constant for formic acid (KS1U1B1aS1U1B0 = 1.8 × 10S1U1P1-4S1S1P0), we can calculate the concentration of H+ ions and then convert to pH.
KS1U1B1aS1U1B0 = [HCOO-][H+]/[HCOOH]
1.8 × 10S1U1P1-4S1S1P0 = (x)(x)/(0.0667 - x)
Assuming x is small compared to 0.0667, we can simplify the equation to:
1.8 × 10S1U1P1-4S1S1P0 = xS1S1P0 2 /0.0667
x = 1.8 × 10S1U1P1-4S1S1P0 11ea7e2d_d194_f6dd_a2f7_436db4e25572_TB4940_11 * 0.0667 / 2
x = 6.02 × 10S1U1P1-6S1S1P0
pH = -log[H+] = -log(6.02 × 10S1U1P1-6S1S1P0) = 8.22
Therefore, the approximate pH at the equivalence point is 8.44. Choice C is the best answer.

What Is the Approximate PH at the Equivalence Point of a Weak