It sounds like you're asking about a compound that's 50% sulfur and 50% oxygen by mass. This describes sulfur dioxide (SO2).

Here's why:

* Sulfur's atomic weight: 32.06 g/mol

* Oxygen's atomic weight: 16.00 g/mol

The molecular weight of SO2 is 32.06 + (16.00 x 2) = 64.06 g/mol.

* Sulfur's percentage: (32.06 g/mol / 64.06 g/mol) x 100% = 50%

* Oxygen's percentage: (32.00 g/mol / 64.06 g/mol) x 100% = 50%

Therefore, sulfur dioxide (SO2) has a 50% sulfur and 50% oxygen composition by mass.