Question

Titration of Na2CO3 with HCl. (pKa1(H2CO3) = 6.352, pKa2(H2CO3) = 10.329) . Calculate the pH at each of the following volumes of 0.20 M HCl added to 20.00 mL of 0.10 M Na2CO3. I figured out everything except for adding 20.00 ml of 0.20M HCL. Please help asap.

Answer 1

At this point you have a saturated solution of H2CO3. The Internet has some wide and wooly discussions about Ka for H2CO3 and molarity of a saturated solution of CO2. The (CO2) in the room temperature range is between 0.29M and 0.056M. If we use ka1 of 4.7E-7 (and there is some disagreement about that K1 value), the pH comes out to be very close to the values found for H2CO3 so I would go with that.
........H2CO3 --> H^+ + HCO3^-
I........0.029....0......0
C.........-x......x......x
E........0.029-x..x......x
and solve for x.
Although this arrangement may not be exact and the theory a bit fuzzy, the calculations work out to be very close to what is found in practice. I used k1 = 4.7E-7.