To determine the rate constant for the reaction between hydrochloric acid HCl and sodium thiosulfate Na2S2O3 , we can use the rate law equation:rate = k [HCl]^m [Na2S2O3]^nwhere rate is the initial rate of the reaction, k is the rate constant, [HCl] and [Na2S2O3] are the initial concentrations of HCl and Na2S2O3, respectively, and m and n are the orders of the reaction with respect to HCl and Na2S2O3, respectively.Since we are not given the orders of the reaction m and n , we will assume that the reaction is first-order with respect to both reactants. This means that m = 1 and n = 1. The rate law equation then becomes:rate = k [HCl] [Na2S2O3]Now, we can plug in the given values and solve for the rate constant k :8.7 x 10^-5 mol/L.s = k 0.1 M 0.05 M To solve for k, divide both sides of the equation by 0.1 M 0.05 M :k = 8.7 x 10^-5 mol/L.s / 0.1 M 0.05 M k = 8.7 x 10^-5 mol/L.s / 0.005 M^2 k = 1.74 x 10^-2 M^-1.s^-1The rate constant for this reaction is 1.74 x 10^-2 M^-1.s^-1.