Question

In: Chemistry

Suppose you have 50.1 mL of a solution of H3PO4. You titrate it with a 0.10...

Suppose you have 50.1 mL of a solution of H3PO4. You titrate it with a 0.10 M solution of KOH and use 15.4 mL to reach the endpoint.

What is the concentration of H3PO4?

What was the pH of the original H3PO4 solution?

Solutions

Expert Solution

balanced chemical equation :

H3PO4 +   3 KOH ----------------------> K3PO4 + 3H2O

C1 V1 / n1 = C2 V2 / n2

C1 x 50.1 / 1 = 0.10 x 15.4 / 3

C1 = 0.0102 M

concentration of H3PO4 = 0.0102 M

H3PO4 ---------------------> H3PO4- + H+

0.0102                                 0              0 --------------> initial

0.0102-x                             x               x ----------------> equilibrium

Ka1 = x^2 / 0.0102 -x = 6.9 x 10^-3

x^2 + 6.9 x 10^-3 x - 7.07 x 10^-5 = 0

x = 5.64 x 10^-3

[H+] = 5.64 x 10^-3 M

pH = -log [H+]

pH = -log (5.64 x 10^-3)

pH = 2.25

pH of the original H3PO4 solution = 2.25


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