I don't see where the problem is. You have from the definition
alpha = A/l/c
The concentration c cannot be entered in this formula as ppm, of course.
I assume that by ppm you meam ppm in volume of gas.
You need therefore to specifify the pressure and the temperature.
For a standard pressure (1 atm) and temperature, you know that 1 mole occupies 0.022414 m³.
If the pressure or the temperature are not the standard one, you need to use the perfect gas law to calculate the volume of a mole.
In standard conditions, a concentration c=8500ppm means that there is 0.008500 mole of acetylene in a volume of 0.022414 m³. So the concentration is 0.0085/0.022414 mole/m³.
From this you can calculate alpha.
Note that you may need to correct what I did here above ...
because I did not understand what you meant by "... 8500ppm of 1% Acetylene gas ...".
What did you do actually? Can't you explain that clearly?
Should I guess that the absorber gas had a real concentration of 85ppm acetylene ????
As I did not participate in your experiment, you should understand that I need a clear description of what you did.
My guess is that the initial gas in the device was air.
By some unknown mean you manage to introduce 8500 ppm of a gas mixture GM.
This gas mixture GM has a concentration of 1% acetylene (1% in volume, I guess).
If this guess is correct then the concentration of acetylene in the device is 85 ppm, and this value should be used to calculate alpha, instead of 8500 ppm.
99% of a scientific job is good communication (bidirectional)
