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HH Equation to calculate [HCO3] 
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#1
Nov709, 03:52 PM

P: 18

I'm given the following values: pH = 7.48, PO2 = 120mmHg, and PCO2 = 18mmHg.
I have to find the [HCO3] using these values. There's one thing I don't understand about the HendersonHasselbalch equation and that is how to calculate the [CO2]. I found out that [CO2] = 0.03*PCO2. My question is, what is the 0.03 that is multiplied to the PCO2 to get the [CO2] value? One last thing, the pKa in the HH equation, is it of the HCO2? Thanks 


#2
Nov709, 04:07 PM

Admin
P: 23,595

As far as I can tell pO_{2} is irrelevant. You need to find concentration of H_{2}CO_{3} (could be you are to assume it is identical with concentration of dissolved CO_{2}, that's a common approximation). Then you should use HH equation, pK_{a1} describes equilibrium of the reaction
H_{2}CO_{3} <> H^{+} + HCO_{3}^{} and you have it either in your textbook or in any tables with Ka values. 


#3
Nov709, 04:33 PM

P: 18

Yes, I used the HH equation to find the [HCO3] and it came out to be 13. I' not given any concentration for CO2 or H2CO3. In this case, how would you be able to find the concentration of CO2 or even H2CO3, if its possible at all? The value I used for [CO2] was something I found on Cornell's website but it was not explain why they used that, which is what I'm asking.



#4
Nov709, 05:56 PM

Admin
P: 23,595

HH Equation to calculate [HCO3]
Concentration of CO_{2} is something that can be calculated  for known partial pressure  using Henry's law.



#5
Nov709, 06:03 PM

P: 630

Amount of C02 is a linear function of the product P(C02) and aforementioned solubility coefficient. I believe pK is 6.1 in this case. 


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