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March 29, 2017

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I need help please! You have been given 40ml of a 0.6M solution of NaCl and 40ml of a 0.1M solution of KCl. You are also given as much diluent as you need. Show all your work and indicate how you would make 40ml of a single solution that contains 0.005M NaCl and 0.025M KCl.

  • Chem - ,

    I don't know how many constraints you have; for example must this be EXACTLY 40.00 mL? What kind of equipment do you have; e.g., do you have measuring pipets?
    For starters, the easy way, without a bunch of constraints, is as follows:
    How many mols (or millimols) NaCl & KCl do you need? That is
    40 mL x 0.005M = 0.2 millimol NaCl.
    40 mL x 0.025M = 1.0 millimol KCl.
    Since M = mols/L = mmol/mL, then
    mL = mmol/M.
    mL concd NaCl needed = 0.2 mmol/0.6 M = 0.333 mL of the 0.6 M NaCl.
    mL concd KCl needed = 1.0 mmol/0.1 M = 10.0 mL of the 0.1 M KCl.

    So I would add 0.333 mL of the 0.6 M NaCl solution to a 40 mL volumetric flask, add 10 mL of the 0.1 M KCl solution to the flask, add distilled water to the mark and mix thoroughly.

    This may not be an acceptable answer because as far as I know 40 mL volumetric flasks aren't sold. A way around that would be to make the same kind of calculation, use a 50 mL volumetric flask, and throw 10 mL of the final solution down the drain. Another problem is that pipets that measure 0.333 mL are not readily available. That presents a little problem but probably we could work around that. I may be setting up straw men. Let me know if this answer isn't good enough but define any constraints.

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