Wzorowane na: Chemistry HL Paper 1B, specimen 2025, pytanie 1(e). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Informacja do zadań 1a-1g.
An outdoor public swimming pool (a lido) stays open throughout the year. The water is not heated, and in winter, when the pool is used by cold-water swimmers, its temperature falls to about 5 °C. The water is disinfected with chlorine. When chlorine dissolves in water it reacts to form hypochlorous acid, HOCl, which is the species that actually kills microorganisms.
Hypochlorous acid is a weak acid. In water it is in equilibrium with its conjugate base, the hypochlorite ion, , which kills microorganisms far less effectively than the acid molecule does.
The graph shows what percentage of the dissolved chlorine is present as and what percentage as across a range of pH values, at 5 °C.
(i) Deduce the range of pH values over which the disinfecting action of the water is greatest.
(ii) Using the graph, determine the of hypochlorous acid at 5 °C.
(i) pH below «about» 6.5 / pH 0-6.5 «where almost all of the chlorine is present as HOCl» ✔
(ii) = 7.8 ✔
«at the pH where the curves cross» [] = [] «so = [] and pH = » ✔
OR
[] = «when the acid is 50 % ionized» ✔
(i) [1] for any value or range with an upper limit of pH 7.0 or lower. (ii) M1: pKa read from the crossing point of the two curves, accept 7.7-7.9; M2: the justification that at this pH [HOCl] = [OCl−] so that Ka = [H+] (pH = pKa). Award M2 only if a value in the range 7.6-8.0 has been read.
pKa of HOCl rises as the water gets colder, so the answer is not the textbook room-temperature value. Reading the crossing point at 50 % is the whole of the first mark; the second mark needs the reason, not just a number.
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Policz na kartce jak na maturze, odsłoń klucz i porównaj. Potem odhacz, żeby wiedzieć, co masz już za sobą.
ZrobioneOdhaczanie zapisuje się na Twoim koncie, więc wymaga darmowego konta. Zajmuje chwilę i wracasz do tego samego zadania.