Stock Solutions URGENT

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I am changing my concentrations for a project that I found on here called Looking Downstream: Could Nanosilver in Consumer Products Affect Pond Life? There are two stock solutions of colloidal silver that I need to make that are 500 μg/L and 1000 μg/L.
The concentrations go as followed:
0 μg/L will be 500 mL of pond water (control)
5 μg/L will be 495 mL of pond water and 5 mL of the 500 μg/L stock solution
10 μg/L will be 495 mL of pond water and 5 mL of the 1000 μg/L stock solution
25 μg/L will be 475 mL of pond water and 25 mL of the 500 μg/L stock solution

My questions are: 1) Why do some concentrations use the 500 μg/L stock solution and some use the 1000 μg/L stock solution? 2) How would this convert to these measurements instead (which stock solution do I use for each of these measurements and how much)?

0 μg/L will be 500 mL of pond water (control)
10 μg/L will be ____ mL of pond water and ____ mL the ____ μg/L stock solution
15 μg/L will be ____ mL of pond water and ____ mL the ____ μg/L stock solution
20 μg/L will be ____ mL of pond water and ____ mL the ____ μg/L stock solution
30 μg/L will be ____ mL of pond water and ____ mL the ____ μg/L stock solution

[pswiatko]

Hi Kaela11,

My guess is they use different concentrations for ease of use (i.e. calculations and dilutions).

To answer your 2nd question, use the formula C1*V1=C2*V2 and solve for V2. I'd use whatever stock solution you're more comfortable using.

Hope this helps!

Peter S.