The objective of the Bromocresol Green Equilibrium System Lab was to determine if the equilibrium constant, K, was a true constant at constant temperature. To determine this, the value of the constant was found at different concentrations of HIn, HIn-, and at varying pH, which was used to determine the concentration of H+. K was found using the equation K= [HIn]/([In-][H+]). In order to be a true constant, none of the values of K found should differ from the average by more than two standard deviations. The values of K for solutions 1-5 and U were 4.39E4, 4.53E4, 4.23E4, 4.70E4, 6.35E4, and 4.03E4 respectively. The average K for the lab was found to be 4.71E4 and the standard deviation was 8.302E3. The range then that all experimental values of K must fall under is 3.05E4 to 6.37E4. All experimental values of K for this trial fell within the range. Therefore, K can be determined a true constant.
The pH of the solution does not affect the value of the equilibrium constant, K. K is a ratio of the concentrations of the products and the reactants. A change in pH is simply due to a change in the concentration of Hydrogen ions and hydroxide ions. A change in concentration merely causes a shift in equilibrium towards either the products or reactants. It does not however change the value of the equilibrium constant at constant temperature
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Because of this, the experiment was done at constant temperature. If solutions 1-5 had been done at different temperatures, there would not have been a way to determine the effect of the change in temperature on the magnitude of the equilibrium constant. In order to be able to determine the magnitude, the original K value would first need to be determined. Once determined and both temperatures were measured, an experiment could be done to determine the K of the second