In: Chemistry
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?
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