Question

An Element “E” found in the Pegasus galaxy has an atomic mass of 280.6. The most likely

composition (abundancy) is :

A)280E: 50%,282 E: 50%

B)280E: 60%,282E: 40%

C)280 E: 70%,282E: 30%

D)280E: 80%,282E: 20%

E)280E: 90%,282E: 10%

Answer 1

Hi

An element is made of isotopes. Isotopes are compunds of an element with same atomic number but different atomic mass.

The formula for average atomic mass of an element having isotopes is,

Mavg= M1*percentage of the isotope 1 found in nature+ M2*percentage of isotope 2 found in nature

A) M1= 280 50%

M2= 282 50%

so, Mavg= 280*0.5 + 282*0.5 = 281

This is not rhe likely composition.

B) M1= 280 60%

M2= 282 40%

Mavg= 280*0.6+282*0.4 = 280.8

This is not the likely composition.

C) M1= 280 70%

M2= 282 30%

Mavg= 280*0.7+ 82*0.3 = 280.6

This is the composition of the element that exists in that environment.

D) M1= 280 80%

M2= 282 20%

Mavg= 280*0.8+282*0.2 = 280.4

This is not the likely composition.

E) M1= 280 90%

M2= 282 10%

Mavg= 2280*0.9+282*0.1 = 280.2

This is not the likely composition.

So the correct answer is C.