Re: what is the average pressure in a 12 oz. soda can?

It is difficult to give you an exact answer because the pressure depends on 
a couple of different factors - the most important of which are the 
temperature of the soda and the amount of carbon dioxide that was put into 
the beverage in the first place.

To give you a quick example, let's say that the soda was carbonated to 3.0 
volumes of CO2 and it has been sitting in your refrigerator so it's around 
40 degrees F.  The pressure inside the can will be roughly 17 psig (pounds 
per square inch, gauge) above atmospheric pressure.  If you let the can 
warm up on the counter so its temperature increases to 70 F or so, the 
pressure inside the can will have increased to about 36 psig.

If you'd like to learn more about pressure/temperature relationships, try 
researching the Ideal Gas Law in almost any general chemistry reference.  I 
haven't actually done the calculation, but the carbonation is probably 
behaving according to PV=nRT.  Try the numbers I just gave you and see if 
it works out.  ;)

Thanks for your question!

Kieran