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How to find the mass of a complex solution

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  1. Nov 17, 2014 #1
    1. The problem statement, all variables and given/known data

    0.1004g of PbS, 14 mL of HCl, 3mL HNO3 are put in a 100mL volumetric flask, with deionized water filling up the remainder of the volume up to the mark. 5mL of this solution is added to a 250mL vol. flask in addition to 0.3g NaCl, 2.5g HCl, and deionized water filling up the remainder of the volume up to the mark. What is the total mass of the 250mL solution?

    2. Relevant equations

    N/A

    3. The attempt at a solution

    I don't know the volume nor mass of deionized water used in either instance. Don't molecules of certain fluids "blend into each other," taking up less than their respective volumes would have you believe? Mass is conserved, however, of course. The only way I can think of doing this, with the information given, is to assume that the solutions have a density equal to that of water, but that is a simplification (though there isn't much of anything in either solution, so I'd guess the true density isn't far off).
    Any thoughts?
     
  2. jcsd
  3. Nov 17, 2014 #2

    Borek

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    Staff: Mentor

    No way to answer this question exactly - not enough data.

    The final solution will be diluted enough to assume its density is lower than 1.0(some small digit like 2, 3 or 4) g/mL.
     
  4. Nov 17, 2014 #3
    OK, great. Thanks.
     
    Last edited: Nov 17, 2014
  5. Nov 17, 2014 #4

    Borek

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    Staff: Mentor

    Even 1.0 kg/L (which - in terms of significant digits - is much more accurate; not that I condone use of SD, let's say what I mean is 1.00±0.05 kg/L).
     
  6. Nov 17, 2014 #5
    Out of curiosity, why don't you like significant digits?
     
  7. Nov 17, 2014 #6

    Borek

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    Staff: Mentor

    Because they are a lousy way of dealing with accuracy. How are you going to write 1.00±0.02 using significant figures?

    They are a good rule of thumb when it comes to reporting calculation results when dealing with measured values (say you measured sides of a rectangle to be 2.22 and 3.33, reporting surface as 7.3926 is an obvious nonsense), but they don't say anything real about the accuracy of calculated result. That should be calculated separately, and using appropriate methods.
     
  8. Nov 17, 2014 #7
    Oh, I see. Thanks.
     
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