Zadanie 1b, Paper 1B, maj 2026, TZ1, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, May 2026, TZ1, pytanie 1(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
The Salton Sea is a large saline lake in the Colorado Desert, California, USA, whose surface lies below sea level. Its water level has been falling markedly over the last few decades.
Polecenie
(b) A variety of ions are dissolved in the water of the Salton Sea. The concentrations of the major ions are shown in the table below, based on two different sources (both scientific journals).
| Ion | Concentration / g dm−3, based on Source 1 | Concentration / g dm−3, based on Source 2 |
|---|---|---|
| Mg2+ | 27.96 | 31.60 |
| Na+ | 29.89 | 27.50 |
| Ca2+ | 20.04 | 21.00 |
| K+ | 8.21 | 8.85 |
| Cl− | 148.89 | 155.00 |
| Br− | 5.99 | 6.40 |
(i) Suggest two reasons for the difference in the concentration of magnesium ions between the two sources, other than experimental errors.
(ii) Calculate the percentage difference in the concentration of Mg2+ ions between Source 2 and Source 1.
(iii) A student concluded that the most commonly found salts in the Salton Sea are the chlorides of sodium and magnesium. The student created the following table, using the data from Source 1, to test the conclusion. Calculate the missing values (shown as ?) in the table.
| Ion | Concentration / mol dm−3 | [Cl−] required to form chloride salt / mol dm−3 |
|---|---|---|
| Na+(aq) | 1.300 | 1.300 |
| Mg2+(aq) | 1.150 | ? |
| Cl−(aq) | ? | not applicable |
(iv) Comment on the validity of the student's conclusion in (b)(iii), giving a reason.
(v) The concentration of some of the ions was determined using a spectrophotometer and a calibration curve. Describe how a calibration curve is constructed using a standard solution of a salt of the ion.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) Any two of:
the samples were collected at different locations/areas of the lake ✔
the samples were collected in different years/times of year/seasons «as the lake shrinks, its remaining water becomes more concentrated» ✔
different instruments or sampling/analytical methods were used ✔
the samples were collected from different depths ✔
(ii) mean 29.78 «g dm−3»
«percentage difference » 12 «%» (12.2 before rounding) ✔
(iii) Mg2+ requires 2 Cl− per ion: 2 × 1.150 = 2.300 «mol dm−3» ✔
Cl− concentration = 148.89 ÷ 35.45 = 4.200 «mol dm−3» ✔
(iv) ALTERNATIVE 1:
required [Cl−] for NaCl and MgCl2 together = 1.300 + 2.300 = 3.600 mol dm−3, which is less than the 4.200 mol dm−3 present, AND so the conclusion is valid because there is enough chloride «the remainder is consistent with the chlorides of Ca2+ and K+» ✔
ALTERNATIVE 2:
the salts are dissociated into ions in the lake water, AND so it is not valid to talk about which salts are present ✔
(v) Any three of:
prepare at least five solutions of known concentration from the standard solution «e.g. by dilution» ✔
the range of concentrations must include the expected concentration of the sample ✔
set the spectrophotometer to a chosen wavelength «the wavelength of maximum absorbance» ✔
measure the absorbance «or transmittance» of each solution ✔
take at least two concordant readings at each concentration/repeat the measurement for each concentration ✔
plot a graph of absorbance against concentration ✔
draw a best-fit/trend line «through the origin» OR quote a strong R2 value «above 0.99» ✔
Schemat punktowania
(i) [1] each for two distinct reasons from the list; apply the list principle. Do not accept experimental error, uncalibrated instruments or other suggestions based on experimental error, and do not accept 'global warming' alone. (ii) [1] for 12 % (12.2 % before rounding); the percentage difference is taken relative to the mean of the two values. (iii) [1] for each correct missing value; the two values are independent, so ECF does not apply between them. Do not penalize significant figures. (iv) [1] for a judgement that is valid because there is enough chloride (3.600 against 4.200 mol dm-3), OR for the judgement that the argument is not valid because salts exist as dissociated ions in solution; a judgement with no reason earns nothing. (v) [1] each for any three distinct points from the list; apply the list principle.
Komentarz. The percentage difference is taken relative to the mean of the two values; using one source alone as the reference gives a different figure (13.0 %) and is not the expected method.