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Zadanie 1b

5 pktobliczenioweśrednie (szac.)Paper 2, maj 2022, TZ1

Wzorowane na: Chemistry SL Paper 2, May 2022, TZ1, pytanie 1(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.

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Informacja do zadań 1a-1g.

Calcium turnings burn in air to form mainly calcium oxide, CaO. A small amount of calcium nitride, Ca3N2, is also formed according to the equation:

3 Ca(s) + N2(g) → Ca3N2(s)

Polecenie

In an experiment, calcium turnings were heated strongly in a crucible with a lid. The following data were recorded:

MeasurementMass / g
Crucible and lid38.415 ± 0.001
Crucible, lid and calcium turnings before heating41.907 ± 0.001
Crucible, lid and product after heating42.905 ± 0.001

(i) Calculate the amount of calcium, in mol, that was used.

(ii) Determine the percentage uncertainty in the final mass of product.

(iii) Assume that calcium reacts only with oxygen, that the reaction is complete, and that some of the product escaped from the crucible. Deduce the percentage yield of calcium oxide in the crucible.

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Odpowiedź z klucza

(i) mass of calcium «= 41.907 g − 38.415 g» = 3.492 «g»

n(Ca)=3.492 g40.08 g mol−1n(\ce{Ca}) = \dfrac{3.492\ \mathrm{g}}{40.08\ \mathrm{g\,mol^{-1}}} = 0.08713 «mol» ✔

(ii) mass of product «= 42.905 g − 38.415 g» = 4.490 «g» ✔

percentage uncertainty «=2×0.001 g4.490 g×100= \dfrac{2 \times 0.001\ \mathrm{g}}{4.490\ \mathrm{g}} \times 100» = 0.045 «%» ✔

(iii) theoretical mass of CaO «= 0.08713 mol × (40.08 + 16.00) g mol−1 = 0.08713 mol × 56.08 g mol−1» = 4.886 «g» ✔

percentage yield «=4.490 g4.886 g×100= \dfrac{4.490\ \mathrm{g}}{4.886\ \mathrm{g}} \times 100» = 92 «%» ✔

Schemat punktowania

(i) [1] for 0.0871 mol (0.08713). (ii) M1: mass of product by difference; M2: percentage uncertainty from the two weighings. Award [2] for a correct final answer; accept 0.04 % or 0.045 %. (iii) M1: theoretical mass of CaO; M2: percentage yield. Award [2] for a correct final answer; accept any value in the range 91.5-92.5 %. ECF throughout.

Komentarz

The uncertainty of a mass obtained by subtraction is the sum of the uncertainties of both weighings, not 0.001 g.

Umiejętności w zadaniu