[SanderVH]

Hey guys,

While looking through some exams of the last couple of years
I stumbled upon something I can't quite figure out.

"CO₂ is bubbled into H₂O until we have a solution of 50ml 0,1M."
So the reaction I suppose is happening goes as follows :
H₂O + CO₂  --> H₂CO₃.

"Once we have the solution we start adding NaOH (0,1M) until all
reactions are over." So what will happen is :
H₂CO₃ + NaOH --> NaHCO₃
NaHCO₃ + NaOH --> Na₂CO₃

Ok , nothing fancy here. But when I start calculating some points to
map out the path of my curve I notice something weird.

When I add 99ml to the original solution , I end up getting 0,1mmol of
NaHCO3 and 4,9mmol Na2CO3. This is a solution containing a weak acid
with its conjugated base so I use the pH-formula for a buffer. (pKa2 = 10,33)
So pH = 10,33 + log(4,9/0,1) = 12,02.

When I add another 1ml , I end up at the 2^nd stoichiometric point ,
which means I end up with only Na₂CO₃ in my solution so I use the formula
for a weak base (since its pKb only is 3,67).
pH = 7 + 1/2 * 10,33 + 1/2 * log (5mmol / 150ml) = 11,43.

So what basically happens is that while adding a strong base and getting
rid of all "acids" , I still end up with a lower pH than before my SP. I can't explain
how this comes and use all my formulas as given by our lecturer.
I hope you guys can help me cause my finals are this Friday.
(Sorry for my english , I'm from Belgium)

Greetings

[Borek]

Do you know what are limits of the applicability of the second formula? Have you checked if it can be used here?

To some extent both formulas are approximated as used, so there is nothing surprising they don't work.

[AWK]

Check solubility of carbon dioxide in water. You can reach ~0.03 M  at pressure of CO₂ = 1 Atm in RT