Zadanie 1c
Wzorowane na: Chemistry HL Paper 2, May 2026, TZ3, pytanie 1(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 1a-1c.
Metals combine with non-metals to form ionic compounds, many of which are used in industry and in the laboratory.
Polecenie
Some ionic compounds are the product of acid–base neutralization reactions.
(i) State the names of the acid and the base that would produce sodium methanoate, HCOONa.
(ii) Estimate the pH value of an aqueous solution of sodium methanoate.
(iii) Ionic salts can be used as components of acid–base buffers. Determine the pH of a buffer solution containing 0.240 mol dm−3 methanoic acid and 0.310 mol dm−3 sodium methanoate. The pKa of methanoic acid is 3.75. Use section 1 of the data booklet.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
(i) Acid: methanoic acid AND Base: sodium hydroxide ✔
(ii) greater than 7 «accept any value from 7.1 to 10.0; the actual value is about 8.4 for 0.1 mol dm−3» ✔
(iii) ALTERNATIVE 1:
«Ka = 10−3.75 = [H+][HCOO−]/[HCOOH]»
«[H+] = 10−3.75 × 0.240/0.310 =» 1.38 × 10−4 mol dm−3 ✔
«pH = −log(1.38 × 10−4) =» 3.86 ✔
ALTERNATIVE 2:
«pH = pKa + log([HCOO−]/[HCOOH]) = 3.75 + log(0.310/0.240) =» 3.75 + 0.111 ✔
«pH =» 3.86 ✔
Schemat punktowania
(i) [1]: both names needed; accept sodium carbonate or sodium hydrogencarbonate for the base; do not accept formulas only. (ii) [1]: any value from 7.1 to 10.0; do not accept a range that includes pH 7. (iii) Award [2] for the correct final answer; otherwise M1 for [H+] (or the log ratio) and M2 for the pH.
Komentarz
Sodium methanoate is the salt of a weak acid and a strong base, so its solution is alkaline; a common slip is to give the pH of the acid.