[Harmonium]

Hello all, I have been completely discombobulated by this question regarding pH calculation, and was wondering if anyone could help me out?

There is a litre of solution containing 24mM HCO3-, with a PCO2 of 40mmHg. 100ml of 0.1M HCl is added to this solution. Assuming the PCO2 remains constant, what is the pH of the resulting solution?
(Kp for CO2 at 37 deg C = 0.03mM/mmHg; pKa of system is 6.1)

I thought I'd use the Henderson-Hasselbalch equation, so:

pH = 6.1 + log {[HCO3-] / 0.03 PCO2 }

But then I'm not sure how to factor in for the addition of HCl, because this would give me the pH before the HCl is added.

I can't seem to find any examples like this in my textbooks or notes, and my internet searches have as yet been unsuccessful. This seems to pop up time and time again in exams, so I would really appreciate any help on how to go about answering this type of question. 

Thanks in advance!

[Borek]

What reactions take place when you add HCl? Can you apply their stoichiometry to the solution?

[Harmonium]

I'm not very sure about this, but I believe upon adding the HCl, it is buffered by the HCO3-. Since we're adding 100ml of 0.1M HCl, this is adding 10mM which will be buffered by the HCO3-, the conc of HCO3- will be 24-10 = 14mM.

Returning to the H-H equation, that gives me a pH of 6.1 + log [14/(0.03x40)] = 7.17

Is that right, or have I made a silly assumption?

[Borek]

Looks OK to me.

[Harmonium]

Thanks