Calculating HCO3- in Blood Sample w/ 2.6 X 10^-2 mol/L CO2

[lha08]

Homework Statement

A sample of arterial blood was found to contain 2.6 X 10^-2 mol/L dissolved CO₂. The pH of the sample was 7.43. If it is assumed that in solution the CO2 forms H2CO3, what is the HCO3- concentration in this blood sample? (pKa for H2CO3 is 6.1)

Homework Equations

The Attempt at a Solution

so i first used the henderson hasselbalch equation for the equilibrium H2CO3 <-> HCO3- and then 7.43= 6.1 +log (A-)/(HA) and i got to 21.4= (HCO3-)/(H2CO3). Everything up to here i got, but then in the answer is said to let x= [HCO3-] and therefore 21.4= x / ((2.6X 10^-2)-x)...why do we have to subtract the x from the bottom?

 

[Borek]

Seems like dissolved CO₂ means in this context all forms of carbonate present. But I agree wording is ambiguous.

When referring to buffers we commonly say things like "1M acetate buffer" and we mean sum of concentrations of acetic acid and acetate is 1M.

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