Question

As many as ______ different combinations of water are possible from $H_1^1,H_1^2,H_1^3,O_8^{16},O_8^{17},O_8^{17}$ ?

• A. $16$
• B. $5$
• C. $18$
• D. $24$

Answer 1

The correct option is C $18$

Different combinations can be used in the preparation of water. Using these isotopes, a total of 18 combinations are possible. At first it might seem that kinds of water with the chemical structure $H_{2}O$ based on isotope differences only, are 9 in number:

$H_2^1{O}^{16},H_2^2{O}^{16},H_2^3{O}^{16},$
$H_2^1{O}^{17},H_2^2{O}_{17},H_2^3{O}^{17},$
$H_2^1{O}^{17},H_2^2{O}^{18},and{H}_2^3{O}^{18}$

However, there are actually 18 differing isotope kinds of water. This is because each of the two hydrogen atoms in a particular molecule can be a different isotope, giving these 9 additional varieties of water:

$H^1{H}^2{O}^{16},H^1{H}^3{O}^{16},H^2{H}^3{O}^{16},$
$H^1{H}^2{O}^{17},H^1{H}^3{O}^{17},H^2{H}^3{O}^{17},$
$H^1{H}^2{O}^{18},H^1{H}^3{O}^{18},and{H}^2{H}^3{O}^{18}$