# How do I find a concentration from a sample tested on a spectrophotometer?

I have an assignment to calculate the concentration from a sample. The assignment is:

You have 100 µL of RNA. You take 10 µL from the sample and put it in 990 µL of water in the cuvette. The OD is 0.05. Calculate the concetration of RNA in the sample. Calculate how much µL you have to take to make a solution of 100 ng.

Thus far I have succeded to calculate the concentration from the OD. It is 2 µg/µL. I used the formula that I found on the internet : OD*Standard coeff.(40 µg/ml)*Sample dilution, but I cannot figure out how from that I can calculate the solution with 100 ng, because there is not mentioned the final volume for 100 ng, or how to calculate how much I have to take µL from the cuvette. I do not understand where to begin from.

I tried with the formula C1V1:C1V2, but I could not figure that out.

Any help?

The question being asked is, how many uL of your solution do you need to withdraw in order for that volume to contain 100 ng of the RNA. If there are 2 ug/uL in the RNA solution, that is equivalent to 2000 ngm/uL. So if you want 100 ng of RNA, you will need to withdraw $$\ce{\frac{2000 ngm}{1 uL}~=~ \frac{100 ngm}{x ~uL}}$$ solving for x yields an answer of 1/20 uL or 0.05 uL. 0.05 uL of the RNA solution will contain 100 ng of the RNA.