The mass spectrum of an element with two naturally occurring isotopes is shown below.What is the best estimate of the element's atomic mass?
A)10 amu
B)11 amu
C)10.8 amu
D)10.2 amu
E)10.5 amu

Question

The mass spectrum of an element with two naturally occurring isotopes is shown below.What is the best estimate of the element's atomic mass? 
A)10 amu 
B)11 amu 
C)10.8 amu 
D)10.2 amu 
E)10.5 amu

Quizplus · Accepted Answer

The atomic mass of an element is the weighted average of the masses of its isotopes. To calculate this, we multiply the abundance (percentage) of each isotope by its mass and add them together.

Let x be the atomic mass of the element, and let a and b be the masses of the isotopes.

From the spectrum, we see that Isotope 1 has a relative abundance of about 70%, and its mass is about 10.6 amu. Isotope 2 has a relative abundance of about 30%, and its mass is about 11.6 amu.

Setting up the equation:

(0.7)x = (0.7)(10.6 amu) + (0.3)(11.6 amu)
(0.7)x = 7.42 amu + 3.48 amu
(0.7)x = 10.9 amu
x = 10.8 amu

Therefore, the best estimate of the element's atomic mass is 10.8 amu. Answer choice C is correct.

The Mass Spectrum of an Element with Two Naturally Occurring