( ( Mass ( of ( the atom ) ) ) . )
(ROOT (NP (NP (NNP Mass)) (PP (IN of) (NP (DT the) (NN atom))) (. .)))
Mass of the atom.
(ROOT (S (NP (NP (DT The) (NN sum)) (PP (IN of) (NP (NP (DT the) (NNS masses)) (PP (IN of) (NP (NP (DT the) (NNS atoms)) (PP (IN in) (NP (DT the) (NN formula)))))))) (VP (VBZ is) (VP (VBN referred) (PP (TO to) (NP (RB as) (JJ molecular) (NN mass))))) (. .)))