( ( Percent ( Composition ( ( A ( direct result ) ) ( of ( ( the law ) ( of ( constant composition ) ) ) ) ) ) ) ( ( is ( that ( ( ( each element ) ( within ( a compound ) ) ) ( can ( be ( expressed ( as ( a ( mass percent ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NN Percent)) (VP (VBG Composition) (NP (NP (DT A) (JJ direct) (NN result)) (PP (IN of) (NP (NP (DT the) (NN law)) (PP (IN of) (NP (JJ constant) (NN composition)))))))) (VP (VBZ is) (SBAR (IN that) (S (NP (NP (DT each) (NN element)) (PP (IN within) (NP (DT a) (NN compound)))) (VP (MD can) (VP (VB be) (VP (VBN expressed) (PP (IN as) (NP (DT a) (JJ mass) (NN percent))))))))) (. .)))
Percent Composition A direct result of the law of constant composition is that each element within a compound can be expressed as a  mass percent.
(ROOT (S (NP (DT The) (NN term) (NN percent) (NN composition)) (VP (VBZ means) (NP (DT the) (NN percentage)) (PP (IN by) (NP (NP (NN mass)) (PP (IN of) (NP (NP (DT each) (NN element)) (PP (IN in) (NP (DT a) (NN compound)))))))) (. .)))