Question

An element X has the following isotopic composition ²⁰⁰X = 90 %, ¹⁹⁹X = 8 % and ²⁰²X = 2 %. The weighted average atomic mass of the element X is closest to
(a) 201 u
(b) 202 u
(c) 199 u
(d) 200 u

Answer 1

Answer ⇒ Option (d). 200 u. 


Explanation ⇒

Mass of the Isotopes of the X are 200, 199 and 202. 

% composition of the Isotopes on the Earth are ⇒
 ²⁰⁰X = 90 %, 
 ¹⁹⁹X = 8 % and
  ²⁰²X = 2 %.

∴ Average atomic mass = (90 × 200 + 8 × 199 + 202 × 2) ÷ 100
 = (18000 + 1592 + 404) ÷ 100
 = 19996/100
 = 199.96 u.

The atomic mass obtained is closest to the 200 u, Hence, Option (d). is correct.


Hope it helps.