If 0.04625 mg. of Uranium-235 remains after 2.8 x 10^9 years, what was
the original mass of the sample of Uranium-235? The half-life of Uranium-235 is 7.0 x 10^8 years.

The original mass of the sample of Uranium-235 is 0.7308 mg

Explanation:

Expression for rate law for first order kinetics is given by:

where,

k = rate constant

t = age of sample

a = let initial amount of the reactant

a - x = amount left after decay process  

a) for calculating rate constant

b) for time of years

The original mass of the sample of Uranium-235 is 0.7308 mg

20 drops hope this

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.