Question

1. Sketch the titration curve of 100.0mL of 0.10M H₂CO₃ being titrated with 0.10M NaOH.

a. what is the pH after the addition of 30.0mL of NaOH?

b. What is the pH after the addition of 100.0mL of NaOH?

c. Calculate the concentration of H₂CO₃ at the 2nd halfway point?

Answer 1

a)

mmol of base = MV = 0.1*30 = 3

mmol of acid = MV = 0.1*100 = 10

mmol of acid left after reaction = 10-3 = 7

mmol of conjguate HCO3- formed = 0+7 = 7

this is a buffer so

pH = pKa + log(HCO3-/H3CO3)

pH = 6.37 + log(7/3)

pH = 6.737

b)

this is first equivalence point

pH is between two ionizaiton points

pH = 1/2*(pKa1 + pKa2)

ph = 1/2* ( 6.37 + 10.32)

pH = 8.345

c)

the p Hin the 2nd half way point --> pH = pKa2 = 10.32

then

[H2CO3] when pH = 10.32

pH = pKA 1+ log(HCO3-/H2CO3)

10.32 = 6.37 + log(0.05 / H2CO3

[H"CO3] = 0.05 / (10^(10.32-6.37))

[H2CO3] = 0.00000561 M approx