Question

Average atomic mass of magnesium is 24.31 amu. This magnesium is composed of 79 mole % of Mgand
remaining 21 mole % of 25 Mg and 26Mg. Calculate mole % of 26Mg.
A) 10
B) 1
C) 15
D) 16​

Answer 1

answer : option (A) 10

explanation : average atomic mass of Mg = 24.31 amu

it is given that percentage composition of 24Mg = 79 %

percentage composition of 25 Mg = x % (Let)

and then percentage composition of 26Mg = (21 - x) %

now, atomic mass = 79 % of 24Mg + x% of 25Mg + (21-x)% of 26Mg

or, 24.31 = 79 × 24/100 + x × 25/100 + (21 - x) × 26/100

or, 2431 = 79 × 24 + 25x + (21 - x) × 26

or, 2431 = 1896 + 25x + 21 × 26 - 26x

or, 2431 = 2442 - x

or, x = 11

hence, (21 - x) = 10

so, percentage composition of 26Mg = 10%