Zadanie 4c
Wzorowane na: Chemistry HL Paper 2, May 2026, TZ1, pytanie 4(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 4a-4d.
Sodium hydroxide, NaOH, can neutralize organic acids.
Polecenie
The titration curve below was obtained when the sodium hydroxide solution was added to 25.0 cm3 of the unknown organic acid in part (b).
State, with a reason, which indicator would be appropriate for this titration, using section 18 of the data booklet.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
phenolphthalein ✔ «or phenol red»
AND the pH range of the indicator «8.3–10.0 for phenolphthalein» lies within the steep part of the curve/includes the pH at the equivalence point «about 8.8, in the rapid change of pH around 20.0 cm3» ✔
Schemat punktowania
[1] one indicator from section 18 together with a reason that links the equivalence point (the region of rapid pH change) to the pH range of the indicator. Accept phenolphthalein, or phenol red if the reason refers to its range (6.8–8.4) falling within the vertical part of the curve. Do not accept methyl orange, bromophenol blue, bromocresol green or methyl red, whose ranges lie below the steep part, or an indicator named without a reason.
Komentarz
The equivalence point of a weak acid titrated with a strong base lies above pH 7, so bromothymol blue and the indicators of low pH range do not fit.