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Thread: concentration

  1. #1 concentration 
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    how do you make up a solution with a concentration of 0.25g/l?


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  3. #2  
    Moderator Moderator AlexP's Avatar
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    This sounds like homework and we will not do it for you. If you put effort into it first then we may help you. Show us what you've done or what you try to do.


    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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  4. #3  
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    Urm, 1 part solvent, 3 parts liquid (25% is solid)

    I think...

    But it gets more complicated if you know the stuff...
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  5. #4  
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    Find the density of the solute.
    Find the volume that 0.25g will occupy.
    Remove this volume from 1dm≥ of water.
    Add the 0.25g of solute.
    Done.

    Too late for hour homework? Oh dear....
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  6. #5  
    Forum Masters Degree Numsgil's Avatar
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    Are you sure? Does 1 mL of salt dissolved in 1 liter of water give a volume of 1001 mL? I would think that the dissolved salt would "hide" in between the water molecules and not overly change the volume. After all salt water has a different density from pure water...
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  7. #6  
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    you add 0.25g of solute into 1L of water. solute almost doesn't change the volume of solution. they do "hide", salts disproportionate in water to anions and kations, which are then "covered over" with water molecules
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  8. #7  
    Reptile Dysfunction drowsy turtle's Avatar
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    Quote Originally Posted by Firestarter
    you add 0.25g of solute into 1L of water. solute almost doesn't change the volume of solution. they do "hide", salts disproportionate in water to anions and kations, which are then "covered over" with water molecules
    This is not true. Try filling a cup with water, then add sugar to it. I'm betting it will overflow, even if all the sugar dissolves.

    The concentration means that in every litre of solution, there is 0.25g of solute. Not per litre of solvent.
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  9. #8  
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    no. accually the mass concentration is measured in mass of solute/volume of solution. at least that is how i was tought.
    trust me, the mistake you make with leaving out change of volume with 0.25g/L is very very small.
    it will overflow if you put a lot of sugar in it... because the delta V is not equal zero. but for solid solutes the relative mistake is acceptable.
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  10. #9  
    Reptile Dysfunction drowsy turtle's Avatar
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    Quote Originally Posted by Firestarter
    no. accually the mass concentration is measured in mass of solute/volume of solution. at least that is how i was tought.
    trust me, the mistake you make with leaving out change of volume with 0.25g/L is very very small.
    it will overflow if you put a lot of sugar in it... because the delta V is not equal zero. but for solid solutes the relative mistake is acceptable.
    But it is, however, a mistake.

    If you're doing a titration, this inaccuracy is indeed relevant, therefore the volume of the solute should always be included.
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  11. #10  
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    Accuracy matters I guess. I was trying to figure out how you could be so accurate as to remove the volume displacement which to match the salt.

    Then I realized that how itís really done, day to day, is :

    0.25 g of solute would be weighed out, washed into say for example 250 ml of solvent. This would be placed in a volumetric flask and the beaker washed out a few times with solvent, all washings placed into the volumetric flask. Subsequently the volume would be brought up to the mark for 1000 ml. In this case, volume displacement should have been taken into account without me even realizing it.

    Am I right?
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  12. #11  
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    hehe. in my books that's the equasion for mass concentration.
    do what you want. with 0.25g per liter the relative mistake in volume is around 0,1% most. That is ok for most mortals. and i suppose that "zxcv" is one of them
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  13. #12  
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    Quote Originally Posted by goodgod3rd
    Accuracy matters I guess. I was trying to figure out how you could be so accurate as to remove the volume displacement which to match the salt.

    Then I realized that how itís really done, day to day, is :

    0.25 g of solute would be weighed out, washed into say for example 250 ml of solvent. This would be placed in a volumetric flask and the beaker washed out a few times with solvent, all washings placed into the volumetric flask. Subsequently the volume would be brought up to the mark for 1000 ml. In this case, volume displacement should have been taken into account without me even realizing it.

    Am I right?
    Yes. This is how I do it as well, when making a standard solution. I just had a sort of mental block when I made the post =D

    Quote Originally Posted by firestarter
    hehe. in my books that's the equasion for mass concentration.
    do what you want. with 0.25g per liter the relative mistake in volume is around 0,1% most. That is ok for most mortals. and i suppose that "zxcv" is one of them
    Maybe. But how about making a solution of 5mol/dm≥?

    It becomes very important indeed, and should always be accounted for for good practice.
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