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Homework Help: Density Problem

  1. Sep 4, 2014 #1
    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. jcsd
  3. Sep 4, 2014 #2


    User Avatar

    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.
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