Zadanie 5a
Wzorowane na: Chemistry HL Paper 2, November 2024, pytanie 5(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Polecenie
In a titration, 25.00 cm3 of a 0.120 mol dm−3 solution of a weak monoprotic organic acid, HA, was neutralized by adding sodium hydroxide solution, NaOH(aq), from a burette. The pH curve obtained is shown.
(i) Determine the concentration of the sodium hydroxide solution, in mol dm−3.
(ii) Use the graph to determine the pKa of HA.
(iii) Outline why bromothymol blue can be used as an acid-base indicator.
(iv) Use section 18 of the data booklet to outline why phenolphthalein is a suitable indicator for this titration.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «n(NaOH) = n(HA) = 0.120 × 25.00 × 10−3 =» 3.00 × 10−3 «mol»
OR
volume at the equivalence point = 15.0 «cm3» ✔
« =» 0.200 «mol dm−3» ✔
(ii) «pKa = pH at the half-equivalence point, 7.5 cm3, =» 4.6 ✔
(iii) «conjugate» acid and base forms of the indicator have different colours ✔
(iv) pH at the equivalence point «about 8.7» lies within the pH range of phenolphthalein, 8.3-10.0
OR
the pKa / end point of phenolphthalein lies within the vertical section of the curve ✔
Schemat punktowania
(i) Award [2] for a correct final answer. (ii) Accept values from 4.5 to 4.7. (iii) Do not accept only the colours in acid and alkali without reference to the conjugate acid-base pair. (iv) [1].