Calculate the unknown concentration of H3PO4 based on coulometric titration data

Question

Calculate the unknown concentration of H3PO4 based on coulometric titration data: Approximately 100 mL of 0.2 M KBr(aq) used for the titration. Based on the experiment data, the average time for ten times coulometric titration of H3PO4 is 49.9 s, and each time 150 uL H3PO4 added.

(Note: Phosphoric acid has at most 3 labile protons; in order to calculate the concentration, two phosphate salts provided to help determine how many protons in H3PO4 are labile. 0.5 g K2HPO4 to make 100 mL solution and reaches to the endpoint immediately. 0.5 g KH2PO4 to make 100 mL solution and reaches to the endpoint at 10.02 s.)

Formula may be used: c = (it)/(nFV).

i = 48.25 mA; F = 96485.3329 C/mol; V= volume of analyte

n is the number of labile protons in H3PO4... Please explain in detail your choice of n using half-reaction equations.

Explanation / Answer

Using the given formula, c = (it)/(nFV).........(a)

i = 48.25 mA = 48.25 x 10^-3 A.

t = 49.9 s

F = 96485 C/mol

V for each time = volume of 10 times /10 = 150 micro L

n = 3

c = (48.25 x 10^-3 x 49.9) / ( 3 x 96485 x 1.5 x 10^-6)

Hence concentration of H3PO4 is, c = 5.545 M

Hope this helps !

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