What is the mass of 15mL sample of mercury?

Look up the density of mercury.
mass = volume x density. Make sure the density is in g/mL. If it isn't you will need to convert it. One place to find density of mercury is at www.webelements.com and click on element #80. I don't think the unit given there is in g/mL. If you have trouble converting please repost.

Actually, it isn't necessary for the density to be listed in g/mL PROVIDED you work the problem and report the correct units for mass. I think I overstated the importance of having the density in specific units.

I don't know i need help