Here's how to calculate the percentage by mass of sulfur in the compound:

1. Set up the percentage formula:

Percentage by mass = (Mass of sulfur / Mass of compound) * 100%

2. Plug in the given values:

Percentage by mass = (16.20 g / 70.00 g) * 100%

3. Calculate the result:

Percentage by mass = 0.2314 * 100%

Percentage by mass = 23.14%

Therefore, the percentage by mass of sulfur in the compound is 23.14%.