Question

A sample of water is vaporized and tested in a mass spectrometer. There are $3$ isotopes of hydrogen, ${}^{1}H{,}^{2}H$ and ${}^{3}H$. There are $2$ isotopes of oxygen, ${}^{15}O$ and ${}^{16}O$.
How many peaks would be expected to be found on the mass spectrum?

• A. $3$
• B. $4$
• C. $5$
• D. $6$

Answer 1

The correct option is C $5$

Various isotopes of water with different mass possible are:

${}^1{H}^1{H}^{15}O\left(17\right)$

${}^1{H}^1{H}^{16}O\left(18\right),{}^1{H}^2{H}^{15}O\left(18\right)$

${}^1{H}^2{H}^{16}O\left(19\right),{}^1{H}^3{H}^{15}O\left(19\right),{}^2{H}^2{H}^{15}O\left(19\right)$

${}^1{H}^3{H}^{16}O\left(20\right),{}^2{H}^3{H}^{15}O\left(20\right),{}^2{H}^2{H}^{16}O\left(20\right)$

${}^3{H}^3{H}^{15}O\left(21\right),{}^2{H}^3{H}^{16}O\left(21\right)$

${}^3{H}^3{H}^{16}O\left(22\right)$

Therefore total of 5 peaks will be present in the mass spectrum.

Hence, option $C$ is correct.