Hey there! Sign in to join this conversationNew here? Join for free
Turn on thread page Beta
    • Thread Starter
    Offline

    12
    ReputationRep:
    The formation constants of the [Au(NH3)4]2+ and [Au(S2O3)2]3- species are 1.00*1026 and 1.00*1028, respectively. Consider a leeching solution, in which the equilibrium concentrations of the species are as follows:

    [S2O32-]=0.100 M; [NH3]=0.100 M; total concentration of gold (I) species = 5.50*10-5 M

    Calculate the percentage of gold (I) ions that are complexed with thiosulphate.

    -----

    My attempt:

    I have to calculate [[Au(S2O3)2]3-] and if I can do that then [[Au(S2O3)2]3-]/CAu will bring me my percentage (CAu=5.50*10-5 M given).

    So first we write down the reactions:

    Au+ + 4NH3 -> [Au(NH3)4]+
    Au+ + 2S2O32- -> [Au(S2O3)2]3-

    Now I'll try to get some equilibrium out of here which contains only concentrations I have or can express in terms of constants I have and the one variable I want - [[Au(S2O3)2]3-].

    So there's the trouble. My initial instincts would be "get rid of the [Au+] - you're not given it anywhere" and then regard CAu with CAu=[[Au(S2O3)2]3-]+[[Au(NH3)4]+] being the Au mass balance in terms of the total concentration, which is known. Then rearrange this so that it is an expression in terms of just the one variable we want: CAu-[[Au(S2O3)2]3-]=[[Au(NH3)4]+].

    Then I could subtract the first reaction from the second (K(overall)=K2*K1-1, removing the Au+ which I don't know, and solve for just [[Au(S2O3)2]3-].

    Only one hitch. I can remove Au+ from consideration of the equilibrium by eliminating it through my reaction combination - that is why I chose to subtract - but I cannot remove it from the problem, because it is also present in CAu. My original expression was wrong: it must be CAu=[[Au(S2O3)2]3-]+[[Au(NH3)4]+]+[Au+] to note that there might be some uncomplexed gold (I) ions.

    So how do I do this then?
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: April 12, 2013
Poll
“Yanny” or “Laurel”

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.