[spacebee]

Fundamentally I think there is something I am not understanding when it comes to ppmv and converting to concentration. I am studying for my PE and one of the questions posed by our instructor asks for concentration in ppmv as the final result with calculating partial pressure as an intermediate step. The question and his work through are attached to this post. My work through (which picks up after calculating the partial pressure) is also attached.

I do not understand why my solution is wrong (I used Henry's Law and the equation for converting concentration to ppmv, both of which are shown below), nor do I understand how the teacher is converting from partial pressure to ppmv without some application of the ideal gas law.

Henry's Law: partial pressure = henry's law constant * concentration      OR      Pi = h*Cal      in my work
Concentration (ug/m^3) = (ppmv*molecular weight*10^3)/(24.5)      @ 298K

I thought I could take the partial pressure and insert into Henry's law to get the concentration and then plug that into the ppmv conversion equation and get the right answer but apparently I have done something wrong. Henry's law output the concentration in mol/m^3 so I used the molecular weight to get it into ug/m^3 to use in the second equation.

It doesn't seem to me like the answer is take the partial pressure over what the teacher is calling "total pressure" (760mmHg) multiplied by 10^6  to get the concentration in ppmv.

Any help is greatly appreciated

[mjc123]

What physical situation does Henry's law refer to? Is that applicable here?

Your teacher's method is correct. Remember that ppmv is not, strictly, a concentration, but a volume fraction, and for ideal gases this equals the mole fraction, as the molar volume of all ideal gases is the same at the same T and P, and the mole fraction equals the ratio of partial pressure to total pressure for ideal gases.