Question

X, Y, and Z are three atoms belonging to the same element. The atoms have mass numbers in the order X < Y < Z. They differ in their number of neutrons by one unit and they exist in the ratio of 1 : 1 : 2. What is the average atomic mass of this element if the atomic mass of the lightest isotope is A?

• A. $A+\frac{5}{4}$
• B. $A+\frac{4}{5}$
• C. 4A + 5
• D. 5A + 4

Answer 1

Answer: (A)

$A+\frac{5}{4}$

The atomic mass of X, Y, Z are respectively $A,A+1,A+2$

Average atomic mass $=\frac{1}{4}\times A+\frac{1}{4}\times (A+1)+\frac{1}{2}\times (A+2)=A+\frac{5}{4}$