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Simplified 2.0 mL .20M X added to 9ml

  1. Jun 27, 2010 #1
    Simplified.... 2.0 mL .20M "X" added to 9ml

    1. The problem statement, all variables and given/known data

    Simplified....

    2.0 mL .20M "X" added to 9ml making an 11mL soln.

    What would be the concentration of "X"?


    2. Relevant equations

    M= molarity=moles/liters



    3. The attempt at a solution

    2.0ml=.002L x .2M= .0004 moles/.011L=.0363636...M "X"

    Right?...

    Thanks,

    Warren
     
    Last edited: Jun 28, 2010
  2. jcsd
  3. Jun 28, 2010 #2

    danago

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    Gold Member

    Re: Concentrations

    Sure about that? :smile:
     
  4. Jun 28, 2010 #3
    Re: Concentrations

    Now I am....

    .0363636....
     
  5. Jun 28, 2010 #4

    Borek

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    Staff: Mentor

    Re: Concentrations

    It suggests correct thinking, but writing 2 mL=0.0004 moles and asking if it is correct is... funny. It is about as correct as writing

    2=7

    5=3.14

    and so on.

    Compare dilution and mixing calculations.
     
  6. Jun 28, 2010 #5
    Re: Concentrations

    Molarity = moles/liters so to revert to moles i would multiply by liters....
    .2M x .002 liters = .0004 moles

    Is this the problem?

    Mr. Borek, where is it stated .2ml = .0004 moles?

    Let's re establish the idea:

    .002L of .2M "X" would by definition = .0004 moles "X" ... right?
    The .0004 moles "X" are added to "Y" to create a .011L soln creating a .0004/.011 (moles/liter=molarity) = .0363636....M "X"... right?

    Since I've been confused by the above responses could some one confirm or deny my results and suggest a formula to obtain the correct one if I am wrong?

    Thanks,

    Whalstib
     
    Last edited: Jun 28, 2010
  7. Jun 28, 2010 #6

    danago

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    Gold Member

    Re: Concentrations

    Borek was commenting on the way you set out your working, not the actual math and chemistry involved.

    If your first line was "2.0ml=.002L", then it would be ok; but you instead had "2.0ml=.002L x .2M". 2mL is indeed equivalent to 0.002L, but it is not the same thing as 0.0004 moles. If you ever use an equality sign in your working, you should make sure that the quantities on either side have the same dimensions.

    If i was doing this problem, i'd break it into multiple lines:
    2mL = 0.002L
    0.002L * 0.2 mol/L = 0.0004 moles of X
    0.0004 moles / 0.011L = 0.03636 mol/L

    Or something similar.

    EDIT: I must have posted this just after you edited the post above.
     
  8. Jun 28, 2010 #7

    Borek

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    Staff: Mentor

    Re: Concentrations

    My fault with not quoting you exactly, but you started with

    Which is equivalent of stating 2mL=0.0004 mole. That's an obvious shortcut, and notation is awkward - it suggests something you don't mean.

    0.036M is a correct result. And theory behind (which is not different from what you did) is on the page I linked to.

    Note that volumes are not additive, so in general it may happen that 2mL + 9mL are not 11 mL. Unlikely in the case of diluted solutions, and rarely worth of taking into account, but it may hit one day if you ignore it.

    --
    methods
     
  9. Jun 28, 2010 #8
    Re: Concentrations

    Thanks!

    Sorry for the sloppy notation.

    I'll try to work on that as I have LOTS of questions this semester!

    Whalstib
     
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