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Density of Solution and mass percent

  1. Jul 17, 2015 #1
    There are two numericals,

    1. Density of Methanol is 0.793 kg/l. Find molarity. ANSWER is : Desity/ Molar mass = Molarity (UNDERSTANDABLE)

    In other numerical
    2. Concentration of H2SO4 is 98% by mass having density of 1.84gm/ml. Calculate Molarity.
    here they used another formula.

    now my question is, How second question is different from First ? why didnt they took density and devided it by Molar mass to get molarity?
    What I am missing?
     
    Last edited: Jul 17, 2015
  2. jcsd
  3. Jul 17, 2015 #2
    The density of your H2SO4 looks wrong to me.
     
  4. Jul 17, 2015 #3
    Sorry ! Corrected. It was missing decimal.
     
  5. Jul 17, 2015 #4

    RUber

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    I imagine the difference is in the concentration. If H2SO4 is 98% of the concentration by mass, is the 1.84gm/ml the density of H2SO4 or the density of the concentration?
    Perhaps if you posted the formula you are looking at, it would make clear what the example is trying to show.
     
  6. Jul 17, 2015 #5
    Take as a basis 1 liter of solution. What is the mass of this 1 liter of solution? If the mass fraction of sulfuric acid is 0.98, what is the mass of sulfuric acid in the 1 liter of solution? How many moles is that?

    Chet
     
  7. Jul 18, 2015 #6
    Thank you guys for your reply.

    Formula is MOLARITY = (%age * density *10)/ Density
     
    Last edited: Jul 18, 2015
  8. Jul 18, 2015 #7
    Sorry I didnt get what you are implying.
    Here is another way to Present my Doubt. We say that Molarity of Pure water is 55.5 ( which we calculate by using Density as 1)
    Now By giving mass percentage, What extra information is given ?

    SECOND: what is difference between STRENGTH of soln. and DENSITY of solution.
     
  9. Jul 18, 2015 #8

    Bystander

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    Giving you what?
     
  10. Jul 18, 2015 #9
    Molarity

    Well it is simple derivation, By considering mass of solution = 100gm and H2SO4 = 98gm
    then they converted Mass of solution into VOLUME. and converted mass of H2SO4 into moles.
    Once you have both of them, you can get molarity.

    NOW my confusion is ..... Why didnt they simply converted DENSITY ( gm/ml) Into Molarity ( MOLES/ ml)
    what I am missing?
     
  11. Jul 18, 2015 #10

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    Time to back up a couple steps; what is the definition of molarity?
     
  12. Jul 18, 2015 #11
    Moles in Litre Solution
     
  13. Jul 18, 2015 #12

    Bystander

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    ... and, if the density of 98% sulfuric acid is 1.84 g/cm3, one liter contains how many grams?
     
  14. Jul 18, 2015 #13
    I think I got it ?
    If I only consider Density : 1 litres contain 1840 gm ,
    Now this 1840gm will be of weight of (Sulphuric acid + water) ?
     
  15. Jul 18, 2015 #14

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    Yes. How much sulfuric acid?
     
  16. Jul 18, 2015 #15
    Alright ...

    SO 98gm H2SO4 in 100gm solution.
    100 gm solution = 54.3 ml.

    Weight of Sulphuric acid is = 98/54.3 * 1000 = 1804.8 gm
     
  17. Jul 18, 2015 #16

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    Correct. That will be how many moles?
     
  18. Jul 18, 2015 #17
    18.4 moles

    Thanks.
     
  19. Jul 18, 2015 #18
    This would all have been much simpler if you had chosen as a basis of the calculation 1 liter of solution rather than 100 gm of solution (as I suggested in post #5).
    mass of 1 liter of solution = 1840 gm
    mass of H2SO4 = (0.98)(1840)=1804.8 gm
    moles of H2SO4 = 18.4
    moles H2SO4 per liter of solution = 18.4

    Chet
     
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