Ammonia water is just that, mostly free NH3 and water. On adding, my guess is that the exothermic reaction caused the exit of ammonia and lots of
water vapor. In other words, the ammonia gas may be aiding the removal of water from the solution.
Further, the formation of any ammonium sulfate (and/or the hydrogen sulfate) increases the ionic nature of the acid solution, so even if it is more
dilute per the addition of your 10% aqueous ammonia, the activity coefficient may still be elevated.
Of course, my speculation is just theoretical, not experimental in your case, and assumes the absence of any possible measurement error on your part.
[Edited on 16-2-2015 by AJKOER] |