( Concentration ( ( refers ( to ( ( the number ) ( of ( particles ( of ( ( a substance ) ( per ( unit ( of volume ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Concentration)) (VP (VBZ refers) (PP (TO to) (NP (NP (DT the) (NN number)) (PP (IN of) (NP (NP (NNS particles)) (PP (IN of) (NP (NP (DT a) (NN substance)) (PP (IN per) (NP (NP (NN unit)) (PP (IN of) (NP (NN volume)))))))))))) (. .)))
Concentration refers to the number of  particles of a substance per unit of volume.
(ROOT (S (NP (NP (DT The) (NN number)) (PP (IN of) (NP (NP (NNS particles)) (PP (IN of) (NP (NP (DT a) (NN substance)) (PP (IN in) (NP (DT a) (VBN given) (NN volume)))))))) (VP (VBZ is) (VP (VBN known) (PP (IN as) (NP (DT the) (NN concentration))))) (. .)))