Challenge Problem: In Example, we neglected the contribution of nitrous acid to the ionic...

Challenge Problem: In Example, we neglected the contribution of nitrous acid to the ionic strength.

We also used the simplified solution for the hydronium ion concentration,

(a) Perform an iterative solution to the problem in which you actually calculate the ionic strength, first without taking into account the dissociation of the acid. Then, calculate corresponding activity coefficients for the ions using the Debye-Huckel equation, compute a new Ka, and find a new value for [αO1]. Repeat the process, but use the concentrations of H3O+ and NO22 along with the 0.05 M NaCl to calculate a new ionic strength. Once again, find the activity coefficients, Ka and a new value for [^O1]. Iterate until you obtain two consecutive values of [^O+] that are equal to within 0.1%. How many iterations did you need? What is the relative error between your final value and the value obtained in Example with no activity correction? What is the relative error between the first value that you calculated and the last one? You may want to use a spreadsheet to assist you in these calculations.

(b) Now perform the same calculation, except this time calculate the hydronium ion concentration using the quadratic equation or the method of successive approximations each time you compute a new ionic strength. How much improvement do you observe over the results that you found in (a)?

(c) When are activity corrections like those that you carried out in (a) necessary? What variables must be considered in deciding whether to make such corrections?

(d) When are corrections such as those in (b) necessary? What criteria do you use to decide whether these corrections should be made?

(e) Suppose that you are attempting to determine ion concentrations in a complex matrix such as blood serum or urine. Is it possible to make activity corrections in such a system? Explain your answer.

Example