# Homework Help: Density Problem

1. Sep 4, 2014

### ThatDude

1. The problem statement, all variables and given/known data

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 0.4 μg Hg/mL, what
is the total mass in kilograms of mercury in the lake? (The lake
has a surface area of 100 mi2 and an average depth of 20 ft.)

2. Relevant equations

mass = density * volume

3. The attempt at a solution

I tried converting the density to Kg and the volume to cm3 = mL

Mass = density * volume

= ( (0.4 μg)(1 g/ 106 μg) (10-3 kg / 1 g) ) * ( (100 mi2)(52802 ft2 / 1 mi2)(30.482 cm2 / 1 ft2) (20 ft)(30.48 cm / 1 ft) )

My answer is 6 x 105 kg however the textbook says it's 7 x 105 kg.

2. Sep 4, 2014

### Staff: Mentor

0.4 μg Hg/mL is not a density, it is a concentration. Accidentally, it requires the same approach. I got 6x105 as well.