How do I calculate the amount of buffer needed to resuspend siRNA if I need a stock concentration of 20 uM?

Our siRNA is supplied as a dried pellet in nmol quantities. Stock concentrations are calculated as follows: (quantity of siRNA, nmol)/(volume for resuspension, uL) = stock concentration, umol/L. Therefore, solve for the unknown and include unit conversions to make sure units cancel. Example: You purchased 20 nmols of siRNA and would like a 50 uM stock solution. a. equation: (20 nmol)/ ? uL = 50 umol/L b. solve for unknown: ? uL = (20 nmol)(1 L/50 umol) c. unit conversions: ? uL = (20 nmol)(1 L/50 umol)(1 umol/1000 nmol)(10exp6 uL/1 L) d. answer: ? uL = 400 uL Therefore, use 400 uL of a 1X RNase-free buffer to resuspend 20 nmol of siRNA to make a 50 uM solution.