An element has three naturally occuring isotopes: 90.48% is element-20 (mass = 19.992 amu), 0.27% is element-21 (mass - 20.994 amu), and the remainder is element-22 (mass = 21.991 amu). Calculate the average atomic mass of the element, rounded to three decimal places.

Average atomic mass  = 20.180 amu.

Explanation:

Given data:

Abundance of X-20 = 90.48%

Atomic mass of X-20 = 19.992 amu

Abundance of X-21 = 0.27%

Atomic mass of X-21 = 20.994 amu

Abundance of X-22 =  9.25%

Atomic mass of X-22 = 21.991 amu

Average atomic mass = ?

Solution:

Average atomic mass  = (abundance of 1st isotope × its atomic mass) +(abundance of 2nd isotope × its atomic mass) +(abundance of 3rd isotope × its atomic mass)   / 100

Average atomic mass = ( 90.48×19.992)+(0.27×20.994)+(9.25+ 21.991 ) /100

Average atomic mass = 1808.88  + 5.668+203.42 / 100

Average atomic mass  = 2017.968 / 100

Average atomic mass  = 20.180 amu.

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.