Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: ajones51 on September 27, 2006, 07:47:08 AM
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hi everyone,
I have been trying to solve this questioin but am having a bit of trouble and was wondering if someone could try helping me or show me how to do it.
The question is this:
You have been given the task of making a 1.00 L solution of a 1 ppm standard of Na+. You have the following equipment available with the associated absolute errors:
Anaytical Balance: accurate to 0.01g
Volumetric flasks: 100 ml +/- 0.08 mL, 1000ml =/- 0.30ml (5 of each type)
Pipettes: 1ml +/- 0.01 ml, 10 mL +/- 0.02 mL
Beakers: 10 mL, 250 mL, 1000 mL (several)
As much distilled water as you need
1 unopened 10 g bottle of NaCl
The goal of the question is to make a 1 ppm solution solution of a 1 ppm standard of Na+ as accurately as possible given the equipment. You can use the equipment as many times as you want.
I figure that I need to weigh out an amount of NaCl (MW = 58.4437 g) and convert it to grams of Na+ (MW = 22.98977 g). I then make a small concentrated solution using this weighed out amount of NaCl and use a 10 mL pipette to make a small concentrated solution in the 1000 mL volumetric flask. I should then dilute the contents of this flask with distilled water until my total volume is 1.00 L and the concentration is 1 ppm.
I am having trouble figuring just exaclty how to determine how much NaCl I should weight out as the balance is only accurate to .01g. I think that I need to get a nice number for the grams of Na+ such as 1 g or 2 g but I can't think how much Na+ I should be looking to obtain in the beginning. I am also unsure of how much distilled water to dissolve this weighed out NaCl initially to make the inital concentrated solution. I figure once i get this initial concentration I should dilute perhaps either 100 times with the 10 mL pippete but I can't figure out how to setup my intial numbers to ultimately solve the question.
If someone could please help me, that would be greatly appreciated.
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I would weight 1g of NaCl and dissolve it in a 100mL flask. Then you would have a 1.711*10-1 mol/L solution. With a 1mL pipette and another 100mL flask, you end up with 1.711*10-3 mol/L.
Now, using a 1mL pipette, dilute your solution in 100mL to get a 1.711*10-5mol/L solution.
Finally, 1mL pipette and 1L flask, and you have your 1.711*10-8mol/L solution (or 1ug/L solution).
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This is a uncertainty calculation problem.
That is to say you need to "pool" the uncertainty from atomic mass, uncertainty of balance (+-0.01g) , volumetric flask (the kinds and the number of times of V. flask you used to prepare and dilute the solution) and pipette.
So your answer should be 1 +-xxx ppm as Na.