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The rate of a chemical reaction may be dependent on, or independent of, the concentration of one or more reactants. How the rate depends on reactant concentrations is expressed as a rate law. With the Determining a Rate Law: A “Sulfur Clock” Reaction—ChemTopic™ Lab Activity, determine the rate law for the reaction between hydrochloric acid and sodium thiosulfate. First, hold the HCl concentration constant and vary the Na2S2O3 concentration, then do the reverse, measuring reaction times by monitoring the appearance of sulfur.