Sentence:  Very small concentrations of these hydrides should be detectable ; in interstellar gas , <WSD>concentrations</WSD> as low as * * f molecules / * * f may be sufficient , as compared to the * * f hydrogen atoms * * f required for detection of the 21 - cm line . 
Meaning of the word marked with <WSD> tags:  the strength of a solution; number of molecules of a substance in a given volume