Starting with a concentration of an oligonucleotide in µg/l, convert this to a molar concentration (molarity) using the length of the oligonucleotide to calculate its molecular weight.
You should aim to do these calculations in your head or on a scrap of paper (i.e. without a calculator). The following logic may be helpful:
The molecular weight of the oligonucleotide is calculated by multiplying its length by 300.
To convert a concentration in g/l to moles/l (M) we divide by the molecular weight. Hence dividing the concentration in µg/l by the molecular weight gives us the concentration in µmole/l (µM).
This is multiplied by 1000 to convert from µM to nM.
The molarity of the solution is therefore:
(Concentration x 1000) ÷ (oligo length x 300)
Always do a ‘sanity check’ on your answer. Is it the right order of magnitude? Be suspicious of very high or very low values.