Question

NTSE Sample Question Chemistry Oxygen has two isotopes O16 & O18. If the percentage of O16 is 90 then the atomic weight of oxygen will be (a) 16 (b) 16.2 (c) 16.4 (d) None of these

Answer 1

let there be 100 O atoms
among it there will be 90 O 16 atoms 
so total weight of those atoms is 90*16 = 1440 units
rest 10 atoms are of O 18
so weight of these atoms is 10*18 = 180 units
total weight of all 100 atoms = 1440+180 = 1620 units
if weight of 100 atoms is 1620 units
weight of 1 atom should be 1620/100
= 16.2 units

Ans. b) 16.2
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