Answer to Question #123954 in Physical Chemistry for Claudiu

Question

A potentiometric titration of HCl 0,00735M is done with NaOH 0,01127 M in the burette. Both
solutions have the same ionic strength, 0,4 M. The analysis of the titration using the Gran plot yields
a straight line for the acid side with a slope equal to -76480. The basic side line has a slope of
116092. Nernst slope value is 59,16 mV. Obtain the stoichiometric pK of water at that ionic strength.

Answer 1

The dependence of pK of water on ionic strength is described on the graph below. This data is given for NaCl solution.

For value of 0.4 M (mentioned in the text of the assignment) pK is about 13.7. As it decreases gradually it riches the point of 13.8, then 13.9 and up to 14.0.

We titrate HCl with NaOH solution, in this case for equal concentrations (whenever the electrolyte involves only ions with +1 and -1 charges the ionic strength is the same as the electrolyte concentration), the concentration of the product of the reaction (NaCl) drops twice. So for the value of 0.2 M pK of water riches the value of 13.75 according to the graph.