A student is given a solution that may contain a reducing agent. Acidified potassium manganate(VII) is placed in a burette and run into the solution until t...

This question uses acidified potassium manganate(VII) as a self-indicating oxidising agent in a titration, and also tests reading a burette. As long as a reducing agent is present, the purple colour is destroyed; when it just persists, all the reducing agent has reacted.

(a) Method to show a reducing agent is present (any three) [3]: add the acidified potassium manganate(VII) a little at a time to the solution [1]; the purple colour disappears / is decolourised [1]; keep adding until the purple colour no longer fades and just remains [1]; a reducing agent decolourises the manganate(VII) by reducing purple \(MnO_4^{-}\) to almost colourless \(Mn^{2+}\).

(b) The burette reading is 23.5 cm\(^3\) (accept 23.4 to 23.6) [1]. A burette is graduated from 0 at the top downwards, so the reading is taken at the bottom of the meniscus; here the liquid level sits half a division below the 23 mark, giving 23.5 cm\(^3\). Always read to the nearest 0.05 cm\(^3\) at eye level.

(c) At the point where all the reducing agent has just reacted, the colour change is colourless to (pale) purple / pink (the purple colour just stays) [1]. The first permanent tinge of purple means there is no more reducing agent left to destroy it.

(d) A white tile is placed under the flask so that the colour change and end-point can be seen clearly against a white background [1].

(e) The reaction taking place is redox (oxidation-reduction) [1]: the manganate(VII) is reduced while the reducing agent is oxidised.