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Tutorial 9 Notes

Tutorial 9 Notes - Nov 20th 24th Tutorial Notes 1 The...

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Nov. 20 th , 24 th Tutorial Notes 1) The solubilities of AgI and Bi 2 S 3 in water are 1.2x10 -8 mol kg -1 and 1.6x10 -20 mol kg -1 respectively, at 25 o C. (Note: The solubility for Bi 2 S 3 previously posted on the website was incorrect.) Calculate their solubility constants. Is there any significant difference when activity coefficients are ignored? (i) The dissociation reaction for silver iodide is: AgI(s) Ag + (aq) + I - (aq) Therefore if the solubility of this salt is 1.2x10 -8 mol kg -1 then the concentration of each ion is: + - 8 -1 Ag 8 I -1 1.2 10 mol kg 1.2 10 1 mol kg o o b b b b ! ! " = = = " . For monovalent salts, the concentration equals the ionic strength of the solution – this can be used to calculate the mean activity coefficient using the limiting law: ( ) Ag I 8 5 log 1 1 0.509 1.2 10 5.58 10 1.0 z z A I ! ! + " ± " " ± = " = " #" # = " # = Since 1 ! ± = then: ( ) + - + - Ag I Ag I 2 8 16 1.2 10 1.4 10 sp o o K a a b b b b ! ! = = = " = " Therefore the solubility product for AgI is 1.4x10 -16 and there is no significant difference when the mean activity coefficient is ignored. (ii) The dissociation reaction for Bi 2 S 3 must produce a neutral solution. We know that there are 2 Bi ions and 3 S ions produced in this reaction, although it may not be immediately apparent what the charge number might be. However, if the 2 Bi cations are to be charged balanced by 3 Si ions then 2Bi v+ = 3S v- . The lowest integer values that satisfy this relationship are v + =3, v - =2. From its position on the periodic table we know

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that S 2- will have a completely filled p-orbital (and is therefore a relatively stable ionization state). Therefore the dissociation reaction is: Bi 2 S 3 (s) 2Bi 3+ (aq) + 3 S 2- (aq) The solubility s = 1.6x10 -20 mol kg -1 . Therefore the concentration of the ions in this solution are: ( ) 3+ 20 -1 20 Bi -1 2 1.6 10 mol kg 2 3.2 10 1 mol kg o o b s b b !
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Tutorial 9 Notes - Nov 20th 24th Tutorial Notes 1 The...

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