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Zadanie 1a

6 pktobliczenioweśrednie (szac.)Paper 2, maj 2026, TZ2

Wzorowane na: Chemistry SL Paper 2, May 2026, TZ2, pytanie 1(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.

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Polecenie

Hydrochloric acid, HCl(aq), and hydroiodic acid, HI(aq), ionize completely in water, whereas propanoic acid, CH3CH2COOH(aq), ionizes only partly.

25.0 cm3 of 0.200 mol dm−3 HCl(aq) was titrated with sodium hydroxide solution, NaOH(aq).

(i) Calculate the pH of the hydrochloric acid before any alkali is added. Use section 1 of the data booklet.

(ii) Write the equation for the neutralization of the hydrochloric acid by the sodium hydroxide solution.

(iii) Calculate how many moles of NaOH are needed to neutralize all the hydrochloric acid in the flask.

(iv) Calculate the volume of 0.180 mol dm−3 NaOH(aq) that must be added to reach the equivalence point.

(v) Suggest one test, and the result you would expect, that would allow you to tell apart unlabelled solutions of HCl(aq) and CH3CH2COOH(aq) of the same concentration.

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Odpowiedź z klucza

(i) «[H+] = 0.200 mol dm−3» pH = 0.699 ✔

(ii) HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) ✔

(iii) «n(NaOH) = n(HCl) = 0.200 × 0.0250 =» 5.00 × 10−3 «mol» ✔

(iv) «V=5.00×10−30.180=V = \dfrac{5.00 \times 10^{-3}}{0.180} =» 0.0278 «dm3» / 27.8 «cm3» ✔

(v) ALTERNATIVE 1: Method: measure the pH / use a pH meter / universal indicator ✔ Expected result: lower pH / red for HCl OR higher pH / orange or yellow for CH3CH2COOH ✔

ALTERNATIVE 2: Method: add a carbonate or hydrogencarbonate / a reactive metal ✔ Expected result: HCl gives bubbles of gas faster / more vigorously ✔

ALTERNATIVE 3: Method: measure the electrical conductivity ✔ Expected result: HCl has the higher conductivity ✔

ALTERNATIVE 4: Method: measure the temperature change on adding the same volume of alkali / a base / a metal oxide ✔ Expected result: HCl gives the greater temperature change ✔

Schemat punktowania

(i) [1] 0.699; accept 0.70. (ii) [1] balanced symbol equation; state symbols ignored. (iii) [1]. (iv) [1] accept 0.0278 dm3 or 27.8 cm3. (v) [2] M1 a valid method; M2 the expected result, which must match the method and must compare the two acids; accept specific bases or metals in alternatives 2 and 4.

Umiejętności w zadaniu