"Composition" - the different parts which something is made of. What is the chemical composition of sodium chloride? - Sodium and chlorine. What is the chemical composition of hydrochloric acid? - Hydrogen and chlorine. "Chemical component" is only one part of the chemical composition. So hydrogen is the first component and chlorine is the second component of hydrochloric acid.

composition is the total make up of something component is part of the make up :P