Posted by christina on Monday, July 23, 2007 at 5:09pm.
I REALLY REALLY NEED this to be checked by the END OF THE Day since my finals are tomorrow and I need to know if this is the correct way to do this type of question PLEASE TELL ME IF IT IS CORRECT.
I AM posting this again since nobody replied to the other one
The total Carbon dioxide content (HCO3- + CO2) in a blood sample is determined by acidifiying the sample and measuring the volume of CO2 evolved with a van slyke manometric apparatuses. The total concent was determined to be 28.5 mmol/L. The blood pH at 37o was determined to be 7.48. What are the concentrations of HCO3- and CO2 in the blood? pKa= 6.10
I looked up the eqzn for CO2 in blood
and it was
CO2 + H2O=> H2CO3
H2CO3=> H+ + HCO3-
why is there this conversion factor online for the H-H eqzn for this rxn??
pH = 6.1 + log [HCO3-/(0.03)(PCO2)]
I don't think that I have to use this b/c It wasn't in the problem Q but
anyways I'll post what I did and say what I can't figure out..
and another thing HCO3- is a base??
while CO2 is a acid??
tot concent= (28.5mmol/L)/1000= 0.0285mol/L
pH = pKa+ log(cHCO3-/cCO2-)
but since I like the other eqzn better...
[H30+]= 10^-pH= 10^-7.48= 3.31e-8
Ka= 10^-pKa= 10^-6.10= 7.94e-7
After this I don't know what to do with the total concentration of 0.0285mol/L
I think that I get this now but just to make sure...
since the total concent is 28.5mmol/L
I plugged it into the eqzn for the H3O+
Plugged into the eqzn for the total
[HCO3-]= 28.5mmol- 1.139= 27.36mmol
Using that found concent of the HCO3- & CO2 I plugged it into the eqzn to check the answer.
pH= -log(3.30e-8)= 7.48
which was the pH given..but I had read online that usually the concentration of CO2 is larger than HCO3- but that wasn't the case here. Unless that information is wrong...
and CO2 IS the acid right?
while the HCO3- is the base?
b/c if that wasn't the case then the ratio would be flipped around.
I would like if someone looked at my work and see if it's correct...
I'm sorry I can't help other than to say I have looked on the web and found that HCO3- is about 22-26 mmol/L. Your value is close. HCO3- is a base because it is going to H2CO3 which then goes to CO2 + H2O. MANY references on the Internet refer to HCO3- as the base and CO2 as the acid. I suspect, but I don't KNOW, that the 0.03 comes from the relationship between the solubility of CO2 and the partial pressure of CO2 (Henry's Law). There was frequent use of the term PaCO2 but none of the papers defined it. Here is that site.
So it's NOT just a simple buffer eqzn??
That site REALLY DIDNT HELP...it kept talking about biology in relation to this.... ;____; I'm going to pray this isn't on my test...
by the way my teacher said that the acid was the CO2...
but IF IT WAS JUST A SIMPLE BUFFER EQZN...would it look fine EXCLUDING the fact that this COULD BE more complicated??
Like if per say that this was a simple HA simple acid question?
I think it is a simple buffer problem. The problem is that the web sites don't tell us the units they use and they don't make it clear if it is the pressure of CO2 or the concentration of CO2 they are using. They throw that 33 factor in there (the site I linked you to) but they don't say where the 33 came from. I suspect, but I don't know, that this relates pressure of CO2 to concentration of H2CO3 but I don't know that (Henry's Law). And the web sites seemed to use partial pressure of CO2 interchangeably with concentration of H2CO3 and that left me confused. So I just didn't answer it.
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