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11 Dec 2019

Which of the following statements is not correct?

(a) For 1:1 electrolytes, ionic strength equals the formal concentration

(b) At low ionic strength, activity coefficients approach unity

(c) Activity coefficients increase with increasing ionic strength.

(d) Activity coefficients are assumed to be unity for neutral molecules.

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Sixta KovacekLv2

13 Dec 2019

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Water from Thonon, France has the following composition

a) What is the ionic strength of this water

b) What are the activity coefficients for HCO3- and CO32- in this water?

c) Assuming an equilibrium constant for the dissociation of bicarbonate: of 4.68E-11 and a pH of 7.3, what is the equilibrium concentration of CO32- in this water?

Groundwater at a temperature of 250C flows through strata containing abundant fluorite (CaF2) mineralization and becomes saturated with respect to that mineral. The equilibrium solubility product, Ksp, for this reaction is 10^-10.4.

a) Calculate the concentrations of Ca2+ and F- once the water has reached saturation with respect to fluorite assuming that prior to entering the mineralized strata the water had an ionic strength of less than 10-5 mol/L and contained negligible Ca2+ and negligible F-. For this part of the problem, neglect activity corrections. (Note that for each mole of CaF2 that is dissolved, you will get one mole of Ca2+ and two moles of F-.

b) Calculate the ionic strength for the groundwater that has the concentrations of Ca2+ and F- that you determined in part a) assuming that Ca2+ and F- are the only major ions in the groundwater. Then calculate the activity coefficients for Ca2+ and F- at this ionic strength. Were you justified in neglecting activity corrections?

c) Repeat the calculations from part a) but this time including activity corrections for water with the ionic strength you determined in part b). Does the calculation including these activity corrections predict more or less dissolution of fluorite?

d) Repeat the calculations from part a) but this time assuming that the groundwater has an initial ionic strength of 0.01 and an initial concentration of Ca2+ of 0.55 x 10^-3 mol/L. (Note that you will need to use the extended Debye-Huckel equation for activities at this ionic strength. Also, remember that each mole of fluorite that dissolves will add one mole of Ca2+ and two moles of F- to what is already in solution.)

(a) For a pH = 7.05 buffer solution prepared with NaH2PO4 and Na2HPO4, it has an ionic strength of 0.13. Calculate the concentrations of [H2PO4 - ] and [HPO4 2-] in the solution by assuming that all activity coefficients are equal to one. Describe briefly the steps you will take to do the calculation if the activity effect should be considered and the activity coefficients are not equal to one.

(b) Determine the total mass of Na2EDTA.2H2O) (FM 372.24) which must be dissolved, along with 1.10 g of Ca(NO3)2.2H2O (FM 200.13), in 500.0 mL of solution (pH 9.52) to give a calcium ion buffer with pCa2+ = 8.80.

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