How to convert nanogram to picomole?
Average molecular weight (MW) of a ribonucleotide = 340g/mol

Size of RNA = x

MW of RNA = 340*x g/mol

moles of RNA needed = y * 10⁻¹² moles (1 picomole = 10⁻¹² mol)

mass of RNA needed = 340*x*y*10⁻¹² g

Conc of RNA = 10*10⁻⁹ g/µl `(1ng =10⁻⁹g)

Volume needed for y picomoles = (340*x*y*10⁻¹²)/10*10⁻⁹ = 0.034*x*y µl

If your RNA is biotinylated then add the MW of biotin to MW of RNA