Acetic acid and ethanol react to form ethyl acetate and water, like this: HCH3CO2(aq) + C2H5OH(aq) → C2H5CO2CH3(aq) + H2O(l)

Imagine 173. mmol of C2H5CO2CH3 are removed from a flask containing a mixture of HCH3CO2, C2H5OH, C2H5CO2CH3 and H2O at equilibrium, then answer the following questions. What is the rate of the reverse reaction before any C2H5CO2CH3 has been removed from the flask?

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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.