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Homework Help: Molar Concentration

  1. Jan 23, 2010 #1
    1. The problem statement, all variables and given/known data
    How many grams of NaCl (molecular weight=58.5g mole^-1) would you dissolve in water to make a 0.5M NaCl solution with 500mL final volume?



    2. Relevant equations



    3. The attempt at a solutionSince, 1 L of 58.5g NaCl is equal 1M I used this to firstly convert 500ml into liters which is 0.5L and then I said that half 0f 58.5g is 27.25g NaCl and this will give me 0.5 M in 0.5L of water.
     
    Last edited: Jan 24, 2010
  2. jcsd
  3. Jan 23, 2010 #2

    Borek

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    Staff: Mentor

    No, 0.5 mole in 0.5L is not 0.5M.
     
    Last edited by a moderator: Aug 13, 2013
  4. Jan 23, 2010 #3
    So, does the number of moles in NaCl change depending on the amount of water?
     
  5. Jan 23, 2010 #4
    [STRIKE]Yes.[/STRIKE] (Misread that "moles in NaCl"...)
    Here's one way to look at it:

    0.5 M NaCl = 0.5 mol NaCl/1 L solution = ? mol NaCl/0.5 L solution

    For the two fractions to be equal, what number does the numerator have to be?
     
    Last edited: Jan 24, 2010
  6. Jan 24, 2010 #5

    Borek

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    Staff: Mentor

    There is no such thing as "number of moles in NaCl". There is either "number of moles of NaCl" or "number of moles of NaCl in the given volume of the solution".
     
    Last edited by a moderator: Aug 13, 2013
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