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Case study·Standard multi-step (3/5)·No calculator

Difficulty guide

The label describes the problem, not your ability.

1/5 - Foundational

Warm-up setup; checks one basic idea.

2/5 - Routine exam skill

Normal syllabus skill; usually one main rule or interpretation.

3/5 - Standard multi-step

Exam-style problem with multiple steps or a common trap.

4/5 - Hard exam skill

High-end exam problem; combines methods carefully.

5/5 - Challenge

Stretch problem beyond normal exam pressure.

Question

A teacher demonstrates the equilibrium Fe3+(aq)+SCN−(aq)⇌FeSCN2+(aq)\mathrm{Fe^{3+}(aq)+SCN^-(aq)\rightleftharpoons FeSCN^{2+}(aq)}. The product ion is red. In four trials the teacher adds Fe3+\mathrm{Fe^{3+}}, adds SCN−\mathrm{SCN^-}, dilutes the mixture, and adds a reagent that removes Fe3+\mathrm{Fe^{3+}}.
Total: 5 marks

(a) Predict the colour intensity after adding Fe3+\mathrm{Fe^{3+}}. (1 mark)

(b) Predict the colour intensity after adding SCN−\mathrm{SCN^-}. (1 mark)

(c) Predict the effect of removing Fe3+\mathrm{Fe^{3+}}. (2 marks)

(d) State the principle used in all predictions. (1 mark)

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cbse-chemistry-11.u6.t6-2.case.005

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