Wzorowane na: Chemistry SL Paper 1B, May 2025, TZ2, pytanie 1(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Informacja do zadań 1a-1d.
Copper(II) ions,
Portions of the rinse water were then titrated with the standardized EDTA solution in the presence of murexide as indicator. The burette readings are given in the table.
| Trial 1 | Trial 2 | Trial 3 | Trial 4 | |
|---|---|---|---|---|
| Final burette reading / (± 0.05 ) | 24.10 | 23.75 | 25.30 | 22.70 |
| Initial burette reading / (± 0.05 ) | 0.00 | 1.50 | 3.00 | 0.50 |
| Volume of EDTA added / | 24.10 | 22.25 | 22.30 | 22.20 |
(i) Suggest two reasons why trial 1 required a much larger volume of EDTA than the other three trials.
(ii) State the term used for a measured value that lies far from the other values in a set of repeated measurements.
(iii) Suggest why trial 1 should be excluded from the calculation of the mean titre.
(iv) Calculate the percentage uncertainty in the titre of trial 2.
(i) Any two of:
trial 1 was a rapid/rough titration «to find the approximate titre», so the end point was overshot ✔
the jet below the tap was not filled/an air bubble was trapped when the initial reading was taken ✔
the tap was leaking/not closed properly ✔
the conical flask was contaminated/rinsed with the sample «instead of with deionized water» ✔
extra EDTA was added «after the colour change» to check that the end point was complete ✔
(ii) outlier ✔
(iii) including it would make the mean titre «and the calculated concentration» too high ✔
OR
it is not concordant with the other three titres «it differs from them by more than 0.10 » ✔
OR
it would introduce a systematic error into the mean ✔
(iv) «» 0.45 «%» ✔
(i) [2 max], list principle: any two distinct reasons. (ii) [1]; accept anomalous result or anomaly. (iii) [1] for any one reason. (iv) [1] for 0.45 % or 0.4 %; the uncertainty of a titre is 0.10 cm3 because two burette readings are subtracted, so 0.22 % from a single reading scores 0.
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