To calculate the mass of a substance, we can use the formula:

Mass = Density × Volume

Given:

- Density = 0.75 grams per milliliter (g/mL)

- Volume = 15 milliliters (mL)

Mass = (0.75 g/mL) × (15 mL) = 11.25 grams

Therefore, the mass of the 15 milliliter substance with a density of 0.75 grams per milliliter is 11.25 grams.