CHM 320 Lecture 11 Chap 8 1Chapter 8 – Activity continueda A + b B = c C + d DReactants ProductsK = [C]c[D]d/ [A]a[B]bBUT REALLY….K = Acc Add/ AaaAbborK = [C]c γcc[D]d γdd/ [A]a γaa[B]b γbbCHM 320 Lecture 11 Chap 8 2Activity: A = [X]xγxxWe know [X] is the molar concentration (moles/L).γxis the activity coefficient for species X. -0.51 z2√μlog γx= 1 + (α√μ/305)Where: μ = 0.5 Σ cizi2(ionic strength)(ci= molarity of ith ion and zi= charge on ith ion)z = charge of species xα = ionic size (pm)CHM 320 Lecture 11 Chap 8 3CHM 320 Lecture 11 Chap 8 4Points concerning Activity (A):• Concentration of ions (ionic strength, μ)is important.• Charge on ions (z) is important.• Size of ions (α) is important.• “Activity” (A) should be used for exact work.• The activity coefficient (γx)of a species is a measure of the effectiveness with which that species influences an equilibrium. • In very dilute solutions (μis small), this effectiveness becomes constant and the activity coefficient is unity. The activity and molar conc. of the species is equal.CHM 320 Lecture 11 Chap 8 5Problem 1 – Ionic Strength CalculationAssuming complete dissociation of the salts, calculate the ionicstrength, μ, of the following:a) 0.2 mM KNO3b) 0.2 mM Cs2CrO4CHM 320 Lecture 11 Chap 8 6Problem 2 – Activity coefficient calculationWhat is the activity coeffiecient, γ, for the following:a) Al3+when μ = 0.05 M.b) Al3+when μ = 0.003 M.CHM 320 Lecture 11 Chap 8 7Problem 3 – Using activities for solubility problems.Using activities, find the solubility of AgSCN (moles of Ag+/ L in the following:a) In a solution of 0.060 M KNO3.b) In a solution of 0.060
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