Question

What information do you need to calculate the average atomic mass for an element?

Answer 1

You need to know the abundance and mass of each isotope of that element.

Explanation:

Isotopes are different versions of atoms for any given element. There are three isotopes of hydrogen.

Protium is H-1 (has a mass of 1 amu having 1 proton)
Deuterium is H-2 (has a mass of 2 amu having 1 proton + 1 neutron)
Tritium is H-3 (has a mass of 3 amu having 1 proton + 2 neutrons)

The abundance of these isotopes is:
H-1 is 99.9985% of all hydrogen atoms
H-2 is 0.0015% of all hydrogen atoms
H-3 is 0% of hydrogen atoms (because it's an unstable, radioactive isotope)

When you put all this together you calculate an average mass for hydrogen of 1.00794amu.

See how to do this type of calculation by watching this video:

Hope this helps!