Look up the element on the periodic table
Find the value labeled “Atomic Mass” or “Atomic Weight”
Use the number shown (typically with decimals) as the atomic mass
If the problem asks for a specific isotope, use the isotope’s mass from the isotope data (not the periodic table average)
If given percent abundances and isotope masses, compute:
Atomic mass = (mass₁ × abundance₁) + (mass₂ × abundance₂) + …
Convert percent abundances to fractions if needed (e.g., 27.0% → 0.270)
For molecular compounds, use:
Molar mass = (sum of atomic masses of all atoms in the formula)
Atomic mass of each element in the sum comes from the periodic table or isotope data as required