3.1 mol of #sf(Fe^(3+)# is mixed with 3.2 mol of #sf(SCN^-)# and allowed to reach equilibrium, after which 3 mol of #sf([Fe(SCN)]^(2+))# is formed. What is the value of #sf(K_c)# ?

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Answer 1 · 2016-06-14T22:31:18

#K_c=150" ""mol".^(-1)."l"#

I am assuming that you mean #NH_(4)SCN_((aq))#. The #SCN_((aq))^(-)# ion forms a 1:1 complex with #Fe_((aq))^(3+)#

Construct an #"ICE"# table:

#" "Fe_((aq))^(3+)" "+" "SCN_((aq))^(-)" "rightleftharpoons" "[Fe(SCN)]_((aq))^(2+)#

#color(red)"I"" "3.1" "3.2" "0#

#color(red)"C"" "-x" "-x" "+x#

#color(red)"E"" "(3.1-x)" "(3.2-x)" "x#

The question tells us that #x=3#

#:.nFe_((aq))^(3+)=3.1-3=0.1#

#nSCN_((aq))^(-)=3.2-3=0.2#

and

#n[Fe(SCN)]_((aq))^(2+)=3#

The expression for #K_c# is :

#K_c=([Fe(SCN)^(2+)])/([Fe^(3+)][SCN^(-)])#

#:.K_c=(3)/(0.1xx0.2)=150" ""mol"^(-1)."l"#

The question should have specified the temperature as #K_c# depends on this.