Leaving Cert  Chemistry 2013

In an experiment to measure the dissolved oxygen content of a river water sample, a small amount  of a concentrated solution of compound A, followed by a small amount of a concentrated solution  of alkaline potassium iodide (KOH/KI), were added to a bottle filled with the river water. These additions were made using the method shown in the diagram, avoiding the addition of bubbles of air. After both additions the stopper was replaced carefully and the bottle was inverted several times to ensure thorough mixing of the contents. A brown precipitate  was observed at this stage.  About 1 cm3 of concentrated sulfuric acid (H2SO4) was then added, allowing the acid to run down the inside wall of the bottle. Again the bottle was stoppered and inverted several times to ensure thorough mixing.

(a) Why was it important to avoid trapping air bubbles each time the stopper was inserted into the sample bottle and when using the dropper?more oxygen (O2) would dissolve / would increase dissolved oxygen (O2) / prevent more oxygen (O2) dissolving / increase oxygen (O2) concentration / altering oxygen (O2) / bubbles contain oxygen (O2)

(b) Identify compound A.manganese(II) sulfate / MnSO4

(c) What was observed on addition of the  concentrated sulfuric acid followed by the  mixing of the contents of the bottle? a brown (red, orange, yellow, iodine) solution

After the three additions, the thoroughly mixed contents of the sample bottle were titrated in 200 cm3 portions with a 0.02 M solution of sodium thiosulfate (Na2S2O3). The average titre was 9.4 cm3.  The balanced equation for the titration reaction is:  2S2O3(2–) + I2...S4O6(2–) + 2I(–)    

(d) Describe how the burette was rinsed and filled for use in the titrations. rinse with deionised (distilled) water // rinse with a little of the thiosulfate solution // add solution to burette and clamp vertically / ensure burette is vertical //   use a funnel to fill / remove funnel before adjusting to mark / mark at eye-level //   open tap to fill jet (part below tap) //

(e) Name the indicator used in the titrations. freshly-prepared starch solution

(g) What conclusion should have been reached had a white precipitate been observed instead of the brown precipitate after the first two additions of reagents to the bottle filled with river water?  there was almost (virtually) no (negligible, extremely low) dissolved oxygen (O2)  in the water

(h) Kits, designed for use in the field, allow the dissolved oxygen concentration to be measured  immediately on collection of the sample. Why is the immediate determination of dissolved oxygen considered best practice? biochemical (biological) reactions (photosynthesis, respiration, metabolism) occur / action of micro-organisms (bacteria, algae, diatoms)

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