Supposing I boil the same amount of hops in the same wort (same SG, for the same time, let's say one hour), separately into two pans:
Should I expect different amount of alpha acids to be extracted?
I know that IBU will be very different for the two, but will the absolute amount of bitterness extracted be the same?
To say it in another way: If I add 9L of wort to 2., how will it compare to bitterness of 1.?
On a theoretical level, with worts that are completely free of suspended solids, then for the quantity of iso-alpha acids extracted would be the same. In practice, there are some limiting factors, that make scaling down (using relatively more hops in less solution) non-linear:
there is a maximum solubility of iso-alpha acids, around 120 ppm/80 IBU for a 5% abv beer. Stronger beers allow slightly more. (source). Solubility decreases with lower pH so this is self-limiting.
even with no suspended solids, the kettle wall and proteins precipitated during the boil act to keep AAs out of solution. For the same kettle size, a smaller wort will loose a larger percentage of the AAs this way.
And finally, keep in mind that hop bitterness is not just about alpha acids, but also oxidized beta acids, and polyphenols so whether boiling hops in more/less wort would make an appreciable subjective difference to the bitterness would need to be measured empirically.
The best way to know for sure is to do it yourself and compare.
