Which question cannot be answered using scientific methods?

what is the ph of a hydrochloric acid solution?
is sodium hydroxide an acid or a base?
can acids and bases react chemically?
which is the most interesting acid?

some sources quote the lethal dose, for an average 170lbs opiate-naive individual is between 75 and 375 mg

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.