Question

Oops I left a part of the question out:
2. Mercury poisoning is a debilitating disease that is often fatal. In the human body, mercury reacts with essential enzymes, leading to irreversible inactivity of these enzymes. If the amount of mercury in a polluted lake is 215 ng of mercury / mL, what is the total mass in kilograms of mercury in the lake? (To simplify the math, imagine that the lake is a cylinder with a diameter of 0.67 miles and an average depth of 7.8 feet.)

Answer 1

You certainly did; however, I showed you how to work the problem. All you need to do now is to convert miles to cm and feet to cm, calculate the volume of the "cylinder" (pi*r^2*depth) in cc, convert that to mL and plug in mL in the formula I gave you. Here is the link to my earlier response.
http://www.jiskha.com/display.cgi?id=1411524395

Post your work if you get stuck. Google can help you with the conversions.