Question

In: Chemistry

if you have 100 ml of HA with a pKa of 2.91, and 30.00 mL of...

if you have 100 ml of HA with a pKa of 2.91, and 30.00 mL of 0.200M NaOH was needed to reach the equivalence point at pH 7.7. What was the initial pH right before you started the titration?

Solutions

Expert Solution

concentration of acid :

at equivalence point

millmoles of acid = millmoles of base

100 x C = 30 x 0.2

C = 0.06 M

molarity of acid = 0.06 M

Ka = 10^-pKa

Ka = 1.23 x 10^-3

HA ------------------------> H+ + A-

0.06                           0        0 -------------> initial

0.06-x                         x       x ------------- > equilibrium

Ka =[H+][A-]/[HA]

1.23 x 10^-3 = x^2 / 0.06 -x

x^2 + 1.23 x 10^-3 x - 7.38 x 10^-5 = 0

x = 8.0 x 10^-3

[H+] = 8.0 x 10^-3 M

pH = -log [H+]

pH = -log (8 x 10^-3)

pH = 2.10 -----------------------------------> before start titration


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