19.27 mL should be used.
Work Step by Step
This is a density problem. We are given the density of a substance, .7781 g/mL, and asked that if we want 15 grams of the substance, how large should the volume be. To do this, use the density formula, where density=mass/volume. When we substitute in our values, we see that the volume should be 19.27 mL.