What is the mass of 4.5 moles of nitrogen gas

n = m / M

m = n × M

Nitrogen (N2) - a gas is always with an index 2 if we don't have shown how many molecules are there

m (N2) = 4,5 mol × (2×14.007 g/mol)

m (N2) = 4,5 mol × 28.014 g/mol = 126.063g

if you don't solve with the decimals in class just replace 14.007 with 14

also if you haven't learned abour gasses aways having index 2 then:

m (N2) = 4,5 mol × 14.007 g/mol = 63.0315g

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.