Zadanie 3c
Wzorowane na: Chemistry HL Paper 1B, May 2026, TZ3, pytanie 3(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 3a-3c.
A student estimated the acid dissociation constant, Ka, of propanoic acid, CH3CH2COOH, by measuring the pH of a 0.0200 mol dm−3 standard solution of the acid with a pH probe that had been calibrated beforehand. Three trials gave the pH values below.
| Trial | pH (± 0.1) |
|---|---|
| 1 | 3.4 |
| 2 | 3.2 |
| 3 | 3.3 |
Polecenie
Comment on how the uncertainty of the pH probe affects the accuracy of the Ka values obtained.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
an uncertainty of ± 0.1 in pH changes [H3O+] by about 26 %, and Ka depends on the square of [H3O+], so the probe's uncertainty produces a large spread in Ka «the values cannot be accurate» ✔
Schemat punktowania
[1] for stating that the uncertainty of the probe has a large effect on the calculated Ka or that it is too large for an accurate result. 'The probe is inaccurate' alone does not score unless it refers to the effect on the results.
Komentarz
The three trials give Ka values between 8.1 × 10−6 and 2.1 × 10−5 although the pH values differ by only 0.2.