Question

Lead bromide is found in the environment as a thermal decomposition product of tetraethyl lead and organobromine agents. Tetraethyl lead was added to gasoline to improve octane rating and organobromine compounds were added to convert the lead oxides formed on combustion to volatile lead bromides so the lead oxides would not coat the pistons. Consider the solubility of PbBr_2 that has a solubility product. pK_sp = 5.18. (a) SEE THE BEAKER: A beaker contains pure water and solid PbBr_2= What is the concentration of Pb^2+ and Br^- if activity effects are ignored. (b) The ion concentrations are sufficient to impact ionic strength of the solution. i. What is the ionic strength of the solution? ii. What are gamma_Pb^2 + and gamma_Br^- ? Use which ever equation of gamma is appropriate for the ionic strength. iii. What are [Pb^2+] and [Br^-] taking activity effects into account? (c) What is the ionic strength based on the calculation in part 2(b)iii? If the change is sufficient to impact the gamma values, iterate on the calculated concentration until the concentrations [Pb^2+] and [Br^-] are invariant. A spreadsheet is appropriate for the iteration. (d) Compare the values calculated in part a and part c. What is the ratio of part c concentrations to part (a) for Pb^2+ and Br^-? What is the percent increase? (e) See the Beaker: A beaker contains 0.1000 M NaNO_3 and solid PbBr_2. What are [Pb^2+] and [Br^-]? Ignore the concentration of [Pb^2+] and [Br^-] in the calculation of ionic strength and do not iterate.

Answer 1

a)

[Pb²⁺] = 0.0118 M

[Br^-] = 2 x 0.0118 = 0.0236 M

b)

i) The ionic strength of a solution is a measure of the concentration of ions in that solution.It is given as -

ii) Using Extended Debye-Huckel equation -

[ Since is in picometers this term can be neglected]

iii)

Other parts can be completed using these two parts easily.