Question

A solution is prepared that contains 10 mM NaCl, 20 mM CaCl₂, 20 mM BaCl₂, and 50 mM NiNO₃. Of course, all these salts completely dissolve to their respective cations and anions. The pH is 7.0.

(a) Calculate the ionic strength of the solution.

(b) Calculate the activity coefficients for H⁺, Cl^-, and Ba²⁺ (use the Guntenberg approximation because the solutions do, though note there are a few other equations and a graph that can be used at your discretion any other time).

(c) Compare the concentrations and the activities of these three species—what is the percent difference between the concentration and the activity for H⁺, for Cl^-, and for Ba²⁺?

Answer 1

ANSWER - Question a:

The ionic strength of a solution is defined as

where c = molar concentration of ion (mol/L) and z = charge number of the ion

In this solution we have:

• 10 mM NaCl

• 20 mM CaCl₂

• 20 mM BaCl₂

• 50 mM NiNO₃

Then, the ionic strength of the solution is

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ANSWER - Question b:

According to Gutenberg approximation, the activity coefficient is defined as:

where z = charge of ion and I = ionic strength of solution

Then for

• H⁺

• Cl^-

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• Ba²⁺

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ANSWER - Question c:

The activity (A) of any specie es defined as

where [i] = molar concentration of specie and r_i = activity coefficient of specie

Concentration and activity of H⁺

• The concentration of H⁺ is given by the pH

• The activity is

• The percent difference is

Concentration and activity of Cl^-

• The concentration of Cl^- is given by the Cl^- from NaCl, CaCl₂ and BaCl₂

• The activity is

• The percent difference is

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Concentration and activity of Ba⁺²

• The concentration of Ba⁺² is given by BaCl₂

• The activity is

• The percent difference is

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