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Given percent composition, find mass per litre of solution

  1. May 15, 2016 #1
    1. The problem statement, all variables and given/known data
    A 15% by mass solution has a density of 1.2 g/mL. What is the mass per liter concentration of this solution?
    → (a) 180 g/L (this is the answer, pretend you don't know)
    (b) 150 g/L
    (c) 125 g/L
    (d) cannot be determined from this information
    2. Relevant equations
    Density
    Possibly percent composition
    3. The attempt at a solution
    I don't know g/L.
    D=m/v
    I assume 15% of the solution is 15g (This is clearly not the case, or I wouldn't be here)
    1.2=15/v
    v=12.5mL
    Convert mL to L
    15g/0.0125mL
    1200g/L <---Not even on the answer list, and sounds unreasonable anyway

    Without straight telling me the answer, how does one approach a question such as this?
     
  2. jcsd
  3. May 15, 2016 #2

    Charles Link

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    It's not the best problem in my opinion because the solution has a mass of 1200 g per liter, and if you subtract out the material that was dissolved, you have presumably what is water (when you are given a solution and the material is unspecified, water is usually implied), that must have a density that is at least 1020 grams per liter, and the volume might even be slightly less than a liter. (Sometimes the dissolved material will add volume. When it is removed, the volume could decrease.) 15% by mass of 1200 grams is 180 grams. I guess it is also fair to assume that the solution is not a water solution. Anyway, to help you with your calculations, there are 1000 ml (milliliters) in a liter. To do a percentage, you divide by 100. e.g. for 15%, its fractional value is 15/100=0.15.
     
    Last edited: May 15, 2016
  4. May 15, 2016 #3

    SteamKing

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    The problem statement clearly says the density of the solution is 1.2 g/mL. Don't you know that 1000 mL = 1 L?
    This is the mass per liter of solution. It is not the mass concentration, however.
    X% mass concentration of a solution means that:

    the mass concentration per liter = the mass of 1 liter of solution * X / 100
     
  5. May 15, 2016 #4
    Now that I think about it, my work above was kinda silly :p
    Plugging into the equation, I get 180g/L from 1200x(15/100), which is the correct answer (which is just 15% of that number).
    What is mass concentration exactly? Is it simply the the part of the solution we are looking at, which implies the rest of the solution is water/other substances?
     
  6. May 15, 2016 #5

    SteamKing

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    Mass concentration is the amount of a substance dissolved in some sort of solute to make a solution. The solute can be water or any other liquid.

    Concentration can be defined in several different ways:

    https://en.wikipedia.org/wiki/Concentration
     
  7. May 15, 2016 #6
    The mass concentration they are looking for is the mass of solute per liter of solution.
     
  8. May 15, 2016 #7
    Got it. Unfortunately I only get to use Molarity in my current year of chemistry, so it's good that I now know what mass concentration is. Thanks for the help!
     
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