Formic acid (HCOOH) is an acid produced by some stinging insects (e.g., fire ant

Question

Formic acid (HCOOH) is an acid produced by some stinging insects (e.g., fire ants) and plants such as stinging nettles. A student working on a research project obtains a 50.00mL formic solution extracted from stinging nettles and titrates the solution with 12.50mL of a 0.100M NaOH solution. Constants needed are in the data table. From the titration data, what is the concentration of Formic acid (FA) in the solution? What is the PH after the student has added 6.25 mL of the NaOH solution? Calculate the PH at the equivalence point.

Explanation / Answer

a) millimoles of acid = millimoles of base

    50 x C = 12.50 x 0.100

C = 0.025 M

concentration of formic acid = 0.025 M

b)

millimoles of acid = 0.025 x 50 = 1.25

millimoles of base = 0.1 x 6.25 = 0.625

HCOOH + NaOH ------------------> HCOONa + H2O

1.25              0.625                           0                0

0.625              0                                0.625         -

it is half equivalence point here pH = pKa

pKa = 3.74

pH = 3.74

c)

at equivalence point only salt remains ;

salt concentration = 1.25 / (12.50 +50)

                               = 0.02 M

pH = 7 + 1/2 [pKa + logC]

pH = 7 + 1/2 [3.74 + log 0.02]

pH = 8.02

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