[neptunium]

Hi,
So I have a stock solution of 0.4mM 4-Nitrophenol.
I make up 5 different concentrations:
1st one: 100μL 4-Nitrophenol with 300μL water
2nd one: 200μL 4-Nitrophenol with 200μL water
etc...
I then take 125μl of each concentration to use in the experiment - how many moles of 4-Nitrophenol is there in the 125μl sample from the 1st concentration?

Thanks a lot,
Johnny

[billnotgatez]

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Please read forum rules (see red link at page top center).

[neptunium]

Ok, sorry. I think I convert 0.4mM into 0.0004M, then multiply this by, for the 1st concentration, 300μl or 300x10^-6, which gives 1.2x10^-7 moles. I then multiply this by 125μl, or 125x10^-6, to give 2x10^-11, or 2x10^-6 μmols

[neptunium]

Anyone able to confirm my working...?
Thanks

[DrCMS]

Anyone able to confirm my working...?
Thanks

Yep they are wrong.

How many moles are in 100μL of your stock solution?
How much is that diluted by?
If you now take 125μL out of the 400μL you have prepared how many moles was in that 125μL

[neptunium]

In 100μl there's 0.0004x10^-4 moles (if initial stock solution was 0.4mM). This is diluted by 300μl of water. So there's 0.0001x10^-4 moles in this diluted solution. 125μl/400μl = 0.31, 0.31x0.0001x10^-4 = 3.1x10^-9 moles or 3.1x10-3 μmoles 
Can anyone confirm this?
Thanks..

[DrCMS]

In 100μl there's 0.0004x10^-4 moles (if initial stock solution was 0.4mM).

Yes but should be expressed as 4x10^-8

This is diluted by 300μl of water. So there's 0.0001x10^-4 moles in this diluted solution.

Yes and No. Diluting the solution does not change how many moles are present just the concentrations.
If you have 2 apples and put them in a bucket of water or in a lake does that change the number of apples?

125μl/400μl = 0.31, 0.31x0.0001x10^-4 = 3.1x10^-9 moles or 3.1x10-3 μmoles 

125/400 = 0.3125 ≠ 0.31