( ( At ( ( the surface ) ( of ( the ( catalyst particle ) ) ) ) ) ( , ( ( the concentration ) ( ( is ( ( cAs moles ) ( per ( unit volume ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN At) (NP (NP (DT the) (NN surface)) (PP (IN of) (NP (DT the) (NN catalyst) (NN particle))))) (, ,) (NP (DT the) (NN concentration)) (VP (VBZ is) (NP (NP (NN cAs) (NNS moles)) (PP (IN per) (NP (NN unit) (NN volume))))) (. .)))
At the surface of the catalyst particle, the concentration is cAs moles per unit 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))))) (. .)))