Question

Hydrogen is 99% `""^1H`, 0.8% `""^2 H`, and 0.2% `""^3 H`. What is its average atomic mass?

Answer 1

These numbers give an average atomic mass of 1.11 u.

Explanation:

The atomic masses of each isotope are:

`""^1"H" = "1.008 u"`
`""^2"H" = "2.014 u"`
`""^3"H" = "3.016 u"`

The atomic mass of hydrogen is the weighted atomic mass, in which the mass of each isotope is multiplied by its relative abundance.

The isotopic composition of hydrogen given in the problem is incorrect.

However, I shall use it in the calculation.

`0.99 color(white)(ll)× "1.008 u"color(white)(l) = 0.998color(white)(m) "u"`
`"0.008 × 2.014 u" color(white)(l)= 0.016color(white)(m) "u"`
`"0.002 × 3.016 u" color(white)(l)= 0.0060color(white)(l) "u"`
`color(white)(mmmmml)"Total" = 1.11color(white)(ml) "u"`

Note: The correct abundances are 99.98 % `""^1"H"` and 0.02 % `""^2"H"`. These give the correct atomic mass of 1.008 u for hydrogen..