What is the approximate pH at the equivalence point of a weak acid-strong base titration if 25 mL of aqueous formic acid (HCOOH)requires 29.80 mL of 0.3567 mol L^-1 NaOH? K_a =1.8 × 10^-4 for formic acid. A)2.06 B)5.48 C)8.52 D)11.94

Question

Quizplus · Accepted Answer

At the equivalence point, moles of NaOH added = moles of HCOOH initially present. Using the balanced chemical equation for the reaction between NaOH and HCOOH, we can find the number of moles of HCOOH initially present:

HCOOH + NaOH → NaCOOH + H2O

1 mole of HCOOH reacts with 1 mole of NaOH.

0.3567 mol/L x 0.02980 L = 0.01062556 moles of NaOH

Since the volume of HCOOH used is 25 mL = 0.025 L, we can find the initial concentration of HCOOH:

0.01062556 mol / 0.025 L = 0.4250224 mol/L

Now we can use the given acid dissociation constant (Ka) for formic acid to find the pH at the equivalence point. Since HCOOH is a weak acid and NaOH is a strong base, the pH at the equivalence point will be greater than 7 (basic). We can set up the following equilibrium equation:

HCOOH + OH- <--> H2O + HCOO-

At the equivalence point, we have added enough NaOH to completely neutralize all of the HCOOH, so the concentration of HCOOH is now zero. We can use an ICE (Initial, Change, Equilibrium) table to find the concentration of OH- and HCOO-:

Initial: [OH-] = [HCOO-] = 0
Change: [OH-] = x, [HCOO-] = x
Equilibrium: [OH-] = [HCOO-] = x

Since we know the initial concentration of HCOOH and the concentration of HCOO- at equilibrium, we can use the acid dissociation constant (Ka) to find the concentration of OH- at equilibrium:

Ka = [H+][HCOO-] / [HCOOH]

We can assume that the concentration of H+ is negligible compared to the concentration of HCOO-, so:

Ka = [HCOO-]^2 / [HCOOH]

Solving for [HCOO-]:

[HCOO-] = sqrt(Ka[HCOOH])

[HCOO-] = sqrt(1.8 x 10^-4 x 0.4250224)

[HCOO-] = 0.0086948 mol/L

At the equivalence point, the concentration of OH- is equal to the concentration of HCOO-, so:

[OH-] = 0.0086948 mol/L

Taking the negative logarithm (base 10) of this concentration gives the pOH at the equivalence point:

pOH = -log([OH-])

pOH = -log(0.0086948)

pOH = 2.06

Finally, we can find the pH at the equivalence point by subtracting the pOH from 14:

pH = 14 - pOH

pH = 14 - 2.06

pH = 11.94

Therefore, the best choice is C with the explanation provided above.

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