Question

The average atomic mass of a mixture containing 79 mole percent of ${}^{24}Mg$ and remaining 21 mole percent of ${}^{25}Mg$ and ${}^{26}Mg,$ is 24.31. Find the mole percent of ${}^{26}Mg$:

• A. 5
• B. 20
• C. 10
• D. 15

Answer 1

The correct option is C 10

Molecular mass$=\sum x_i{M}_i$

$x_i=$Fraction of the isotope

$M_i=$Mass of the isotope

$\therefore 24.31=0.79\times 24+x\times 25+y\times 26$...........(1)

Also $x+y+0.79=1.00\Rightarrow x+y=0.21$...........(2)

On solving equation 1 and 2 we get,

$\therefore x=0.11$ & $y=0.10$

Thus mole $\%$ of ${}^{26}Mg$ is $10\%$