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Checking my answer/approach - specific gravity of a mixture

  1. Dec 1, 2012 #1
    1. The problem statement, all variables and given/known data
    What is the specific gravity of a mixture 40% ethyl alcohol and 60% water? The specific gravity of ethyl alcohol is 0.79.


    2. Relevant equations



    3. The attempt at a solution
    He showed us a way to do it for another type of problem but I approached it a bit differently and I wanted to make sure my answer was logical/correct before submitting it. So what I did was this:

    Assume 100 mL of mixture. Then you have 40 mL alcohol, 60 mL water.
    Density of water = 1 = mass/60 mL; mass of water therefore = 60 g
    0.79 = density of alcohol/density of water (1), so density of alcohol = 0.79
    I set up a cross multiplication:

    0.79 g/1 mL = x g/40 mL
    I got 31.6 g of ethyl alcohol.

    So the total mass of the mixture is 91.6 g.

    Thus the density of the mixture is 91.6 g/100 mL = 0.916

    Thus specific gravity of the mixture = 0.916/1 = 0.916.

    Does this seem accurate?
     
  2. jcsd
  3. Dec 2, 2012 #2

    haruspex

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    Yes, that's right, if the 60% and 40% were by volume.
     
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