1. Find the molar mass of CuSO₄:

* Cu: 63.55 g/mol

* S: 32.06 g/mol

* O: 16.00 g/mol (x 4) = 64.00 g/mol

* Total molar mass of CuSO₄: 63.55 + 32.06 + 64.00 = 159.61 g/mol

2. Calculate the mass of sulfur in one mole of CuSO₄:

* Sulfur's molar mass is 32.06 g/mol

3. Divide the mass of sulfur by the molar mass of CuSO₄ and multiply by 100%:

* (32.06 g/mol / 159.61 g/mol) x 100% = 20.08%

Therefore, the percent by mass of sulfur in CuSO₄ is 20.08%.