Help why an a concentration of FeSCN^2+ can be assumed to be equal to that of the initial SCN- concentration.?

2019-03-08 7:46 am

Can someone please help me with this it's for my pre-lab assignment and I'm not even sure what it's asking.

Question: The solutions used to construct the calibration curve (Beer's Law plot) for FeSCN^2+ have an excess of Fe^3+ and a known initial concentration of SCN-. Explain why the concentration of FeSCN^2+ can be assumed to equal that of the initial SCN- concentration.

回答 (1)

胡雪8°

2019-03-08 9:40 am

✔ 最佳答案

Equation for the reaction:
Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) …… Kc

As Kc is large, the reaction lies almost to the right. In other words, the limiting reagent (or known as limiting reactant) on the left should almost completely converted to FeSCN²⁺ ion.

Since Fe³⁺ ion is in excess, SCN⁻ ion is the limiting reagent. Therefore, SCN⁻ ion almost completely converted to FeSCN²⁺ ion, and we can assume that the final concentration of FeSCN²⁺ is equal to the initial concentration of SCN⁻ ion.

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