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Mixing solutions - wrong answer?

  1. Jan 20, 2007 #1
    1. The problem statement, all variables and given/known data
    Here is two similar questions but which I got a different answer to the back of the book

    1. You are to prepare 50ml of 2M sulfuric acid, which is to be added to a sample of white wine to make the sulfur dioxide preservative in the wine available in solution, so that its concentration can be determined. What volume of concentrated (18M) acid will you prepare?

    2. A factory discharged 15ML of a liquid waste containing 12ppm (v/v) of a highly toxix compound into a pond. Before it was added, this pond already contained 920ML of wastewater in which this compound was present at a concentration of 0.23ppm (v/v). What was the new concentration of this compound in this pond?


    2. Relevant equations
    conversions


    3. The attempt at a solution
    Q1.
    2 moles/litre, 50ml => 0.1moles

    18moles/litre. Need 0.1moles so 0.1/18=0.0056L=5.56ml but answers suggest 16ml


    Q2.
    ppm (v/v) => 1g/10^6g = 1g/1000kg

    1L=1kg assuming solvent is water

    15ML @ 12ppm => 15*10^6 kg @ 12g/1000kg => 15*10^3 * 12g grams of solute or 180kg of solute

    920ML @ 0.23ppm => 920*10^6 kg @ 0.23g/1000kg => 920*10^3 * 0.23g or 211.6kg of solute

    Hence total solute is 391.6kg. Total solution is 935ML. So a new total concentration of 391.6kg/(935*10^6kg) * 10^6g = 0.42g/10^6g or 0.42ppm. But the answers suggest 0.49ppm.

    Who is wrong?
     
  2. jcsd
  3. Jan 21, 2007 #2

    Borek

    User Avatar

    Staff: Mentor

    1. You are OK.

    2. You are OK and wrong at the same time. Your result is correct numerically, but note that question is about v/v concetration - so you should calculate using L, not g.


    concentration lectures
     
  4. Jan 22, 2007 #3
    v/v => ml of solute per 100ml of solution. Luck in solution of water, 1ml = 1g. I used 1g in my calculations. So all I had to do was to switch g to ml. The magnitude of the numbers wouldn't change. Thanks for picking that up for me. I will be more careful next time.
     
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