This is a useful notion that expresses the fractional departure of a reaction from its commencement to the situation at a particular time. It is a dimensionless ratio that can apply to the decline of a substrate or the appearance of a product. In the latter case, if [P] increases, the extent of the reaction is given by [P]t/[P]ro, where t denotes the time at which the extent of the reaction is determined, and ro implies the concentration of P that pertains at a very long time when the system is no longer changing. For a substrate A which decreases with time, the expression for the extent of the reaction at time t is
([A]0- [A]t )/([A]0- [A]m ), where the subscript 0 specifies the concentration of A at zero-time.
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