How do you calculate rate of reaction?

An experiment to calculate reaction rate with mass/time?

• In my lab report I used pineapple juice and jelly. i kept the jelly in the pineapple juice for 15 minutes straight without any stirring or temperature change. after that i measured the mass of the jelly and it decreased. i know why. but my question is that when i found the reaction rate with the formula of reaction rate = mass/amount of time kept in. and i did that for 5 sets of trials. they all consistently decreased. what does this mean? i don't under stand how the reaction rate decreases but the mass of the jelly decreases at the same time..shouldn't it be increasing reaction rate and decreasing mass of jelly? :S it's urgent! please

• Answer:

  You have to go back to chemical kinetics chapter. The rate of reaction itself is dependent on the concentration of reactants any given instant (unless the reaction is of zero order). So as the mass (an concentration in turn) of jelly decreased so did the rate of reaction.