Compared to the mass of an so2 molecule, the mass of an o2 molecule is (1) One fourth as great
(2) One half as great
(3) the same
(4) twice as great

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.

metric time was an attempt to create a standard unit of time based off of 10, but it never took off an is not included in the metric system (si)