A compound is known to have a free amino group with a pKa of 8.8, and one other ionizable group with a pKa between 5 and 7. To 100 mL of a 0.2 M solution of this compound at pH 8.2 was added 40 mL of a solution of 0.2 M hydrochloric acid. The pH changed to 6.2. The pKa of the second ionizable group is: A) The pH cannot be determined from this information. B) 5.4. C) 5.6. D) 6.0. E) 6.2.

C) 5.6

Explanation:

From Henderson-Hasselbalch equation:

A = Conjugate base, HA = Acid

%A = 0.251 / (0.251 + 0.1) = 20%, hence %HA = 80%

100 mL of a 0.2 M = 0.02 mol

base = 20% of 0.02 = 0.004

Acid = 80% of 0.02 = 0.016

0.004 moles of base will get protonated by .004 moles of H+ from HCl, forming 0.004 moles of acid

0.02mol original acid - .004 mol H+ = 0.016 mol of conjugate base

Hence A/HA = 0.016/0.004

Since the new pH is 6.2, therefore:

it can be observed without changing the identity of the object.

it depends on what substance you're talking about as well as many other factors. in quantum mechanics, the observation of particles affects the observers ability to measure certain properties. for example: you can know the velocity of a particle, but you can't know it's position; or you can know it's position, but not it's velocity. it's like trying to find out where a grain of sand is by throwing other grains of sand at it. once you find out where it is, it's been moved. it's also like trying to find an answer to the question of whether or not a tree falling in a forest makes a sound if nobody is there to hear it. paradoxically, the only way to find out is to be there and hear it.