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To get the atomic mass of an element, we must take a weighted average of the atomic masses of its .

That is, we multiply the atomic mass of each isotope by its percentage expressed as a decimal fraction. Then we add them together.

The atomic mass of hydrogen-1 is 1 u and the atomic mass of hydrogen-2 is 2 u.

The atomic mass of hydrogen is then

$"0.999 85 × 1 u + 0.000 15 × 2 u" = "0.999 85 u + 0.000 30 u" = "1 u"$ (1 significant figure)