Calculation- can't see where I am going wrong???

[shenzys]

The question is:
You are given a 1 ml vial containing a concentrated solution of acetylcholine chloride (RMM = 181.7). A fter diluting the soln. 1000 fold, you add 0,1 ml of it to a 25ml organ bath, to give a final concentrationof Ach of 10nM. Calculate for the orginal stock solution, the millimolar concentration.

I have worked this out like this:

10nM = 0.00001mM
This has been diluted by a factor of (25/0.1) 250. Therefore:
0.00001*250 = 0.00025mM.
This comes from a solution that has been diluted 1000 fold, therefore I would divide that 0.00025 by 1000, however the answer of 2.5*10^-6 is not correct. To get the correct answer, I would need to multiply the 0.00025 by 1000 to get the correct 2.5mM. What have I done wrong???
Thanks for your help.

[Nik]

From your post you know where you went wrong - when working back to get an original conc you always multiply dilution factors.
Where has this 1000 dil factor come from ? You have written a dilution of 100 fold not 1000.

[shenzys]

From your post you know where you went wrong - when working back to get an original conc you always multiply dilution factors.
Where has this 1000 dil factor come from ? You have written a dilution of 100 fold not 1000.

Sorry the question is supposed to be 1000 fold. I have corrected it.

[shenzys]

From your post you know where you went wrong - when working back to get an original conc you always multiply dilution factors.
Where has this 1000 dil factor come from ? You have written a dilution of 100 fold not 1000.

Okay, thank you I understand now...I had to multiply by the dilution factor which was 1000...therefore I would have received the correct answer of 2.5mM. Thanks again.