Question

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)` ?

Answer 1

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

Explanation:

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.