Solve Density Problem: Mercury in Lake | Mass = 6 x 105 kg

[ThatDude]

Homework Statement

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.)

Homework Equations

mass = density * volume

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.

 

[Borek]

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