[dada2]

Hi,
I have a lab report due soon and despite immense effort, I just cant seem to understand how to do the calculations. The experiment reads:

A 5.0 mL vol of 0.002 M SCN- is mixed with 5.0 mL of 0.02 M Fe3+ to form FeNCS2+ complex. The equilibrium concentration is  7.0 x 10^-4 mol/L. Calculate the following:

* moles odf FeNCS2+ that form in reaching equilibrium
*moles of Fe that react to form the FeNCS2+ at equilibrium
*moles of SCN- that react to form the FeNCS2+ at equilibrium
* moles of Fe3+ (unreacted) at equilibrium
*moles of SCN- (unreacted) at equilibrium
*molar concentration of Fe3+ at equilibrium
*Equilibrium constant

I've already calculated the initial moles of SCN- to be 1.0 x 10^-5  and the initial moles of Fe3+ to be 1.0 x 10^-4
I believe the moles of Fe3+ (unreacted) at equilibrium is 9.0 x 10^-5 but then by that method, the moles of SCN- (unreacted) at equilibrium would be 0 which would make the equilibrium constant 0 which doesn't make sense. Any help would be appreciated. Thanks!

[Borek]

I believe the moles of Fe3+ (unreacted) at equilibrium is 9.0 x 10^-5

Why? That'll be if they all reacted to the end, you are given final (equilibrium) concentration.

[dane502]

Hi dada2

A 5.0 mL vol of 0.002 M SCN- is mixed with 5.0 mL of 0.02 M Fe3+ to form FeNCS2+ complex. The equilibrium concentration is  7.0 x 10^-4 mol/L.

The equilibrium concentration of Fe³⁺, SCN^- or FeSCN²⁺?

Assuming that it is equilibrium concentration FeSCN²⁺, convince yourself that the following is true:

[Fe³⁺]_eq = [Fe³⁺]₀ - [FeSCN²⁺]_eq

[SCN^-]_eq = [SCN^-]₀ - [FeSCN²⁺]_eq

where:

• [Fe³⁺]₀ is the inital concentration of Fe³⁺
• [SCN^-]₀ is the inital concentration of SCN^-
• [Fe³⁺]_eq is the equilibrium concentration of Fe³⁺
• [SCN^-]_eq is the equilibrium concentration of SCN^-
• [FeSCN²⁺]_eq is the equilibrium concentration FeSCN²⁺

This way you should not compute any concentration to be 0! 

This goes also for moles - you should not have a hard time rewriting the expressions from computing concentrations at equilibrium to computing amount of substances - moles as you call it - at equilibrium.

[AWK]

_{A 5.0 mL vol of 0.002 M SCN- is mixed with 5.0 mL of 0.02 M Fe3+ to form FeNCS2+ complex. The equilibrium concentration is  7.0 x 10^-4 mol/L.}
The equilibrium concentration of which ion?

Write down a stoichiometry of reaction first