1. Identify the Elements and their Atomic Masses:

* Na (Sodium): Atomic mass = 22.99 amu

* S (Sulfur): Atomic mass = 32.07 amu

* O (Oxygen): Atomic mass = 16.00 amu

2. Count the Number of Atoms of Each Element:

* Na: 2 atoms

* S: 1 atom

* O: 3 atoms

3. Calculate the Total Mass:

* Na: 2 atoms * 22.99 amu/atom = 45.98 amu

* S: 1 atom * 32.07 amu/atom = 32.07 amu

* O: 3 atoms * 16.00 amu/atom = 48.00 amu

4. Add the Masses Together:

45.98 amu + 32.07 amu + 48.00 amu = 126.05 amu

Therefore, the formula mass of Na₂SO₃ is 126.05 amu.