In effect of concentration on rate, why is it important to

havethe same potassiumin four reaction mixtures?

If we change the concentration of Potassium since we take KI solution, concentration of I - changes, then the rate changes accordingly

Rate = k[H2O2 ]^a[I-]^b [H+]^c

The concentrations of I - and H + are held constant in the procedure

thus to study the rate of the reaction, concentration of KI solution has to be constant only the peroxide solution varies.