[MrTeo]

Hello everybody

Here's a problem on which I had some difficulties... especially in linking their request to find the pressure of CO needed to intoxicate a man with the equilibrium expression:

~•~

Hemoglobin can combine with O₂ and CO forming oxi- and carboxyhemoglobin. The equlibrium constant for this reaction:

HemO_2(aq) +CO_(g)⇄HemCO_(aq)+O_2(g)

is 200. An amount of carboxyhemoglobin equal to that of oxyhemoglobin is known to be lethal. Find the partial pressure of CO in the air (at STP) needed for this condition to verify

~•~

Now, the equilibrium expression is (species in aqueous solution have constant vapor pressure):

$$ K=\frac{\left[O_2\right]}{\left[CO\right]} /$$

But to equal the values of HemO₂ and HemCO I need to know the starting concentration of HemO₂ (if we call it c the final concentration of both species will be c/2) and then find the pressure of O₂ and CO using the equilibrium, that's why I think the problem lacks of necessary data.

Any opinion or advice will be greatly appreciated... thanks in advance ^^

[Schrödinger]

There is something that I can think of.

Let's say you started with 'n' moles of HemO₂. Since you have n/2 moles of HemO₂ and n/2 moles of HemCO in the end. You also have n/2 moles of O₂.
Now, maybe you should use the equilibrium constant. But to use it, you need to know the volume of the container right? (if the experiment is carried out in a container).  Only then can you convert moles of O₂ into pressure. So I too think there's insufficient data.

I'm sorry if I am terribly mistaken. I'm just a rookie...

[MrTeo]

Well, considering that the pressure is inversely proportional to volume and that all the compounds in our chemical equation have coefficient 1, I think the volume of the container goes away when you write the equilibrium expression and only the numbers of moles of the two gases remain... anyway I agree with you when you say that there is some data missing, as we can't find out which is the amount of CO needed to intoxicate a man if we don't know how much oxi-hemoglobin was in his blood before. What do you think of this, have you any ideas to pass over this obstcle?

btw... don't worry, I'm a rookie too