Chemistry HL Paper 1B, May 2025, TZ1

Determine the mass of sodium thiosulfate pentahydrate, $\ce{Na2S2O3 * 5H2O}$, needed to prepare 250.0 $\mathrm{cm^3}$ of a 0.2000 $\mathrm{mol\,dm^{-3}}$ solution.

Explain how the 0.2000 $\mathrm{mol\,dm^{-3}}$ solution is prepared in a volumetric flask from the weighed solid.

Suggest how to prepare 100.0 $\mathrm{cm^3}$ of a 0.05000 $\mathrm{mol\,dm^{-3}}$ sodium thiosulfate solution from the 0.2000 $\mathrm{mol\,dm^{-3}}$ solution.

The student's results are shown below. Each time was measured to ± 0.1 s. Concentration of $\ce{Na2S2O3}$ / $\mathrm{mol\,dm^{-3}}$ Time, trial 1 / s Time, trial 2 / s Time, trial 3 / s Time, trial 4 / s Time, trial 5 / s Average time / s 0.2000 ± 0.08 % 17.2…

The average times were plotted against concentration. Draw a curve of best fit through the points on the graph.

A second student repeated the experiment with the same five solutions and the same apparatus and obtained the following times. Concentration of $\ce{Na2S2O3}$ / $\mathrm{mol\,dm^{-3}}$ Reaction time / s (± 0.1 s) 0.2000 ± 0.08 % 20.1 0.1000 ± 0.2 % 37.5 0.0500…

State one safety concern arising from a product of this reaction, and a precaution that should be taken because of it.

Suggest a sequence of experimental steps that would give a pure sample of each of the three components.

The student recorded the following masses. Sample Mass / g (± 0.01 g) Percentage by mass Mixture before separation 6.35 not applicable Iron after separation 2.58 Calcium carbonate after separation 1.71 Potassium chloride after separation 2.31 36.4 % Calculate…

The three percentages in 2b add up to more than 100 %. Suggest an error that would explain this result and how it could be reduced or eliminated.

The results did not support the student's hypothesis. Suggest what the student overlooked when forming the hypothesis.

In a separate experiment, the students needed to separate a mixture of two miscible liquids, ethanol and pentan-1-ol. State the most suitable method of separation and the physical property on which it depends.

Draw a line of best fit on the graph, extrapolating it beyond the range of the data.

State, in mathematical terms, how the absorbance of these solutions depends on their concentration.

The colorimeter can record either absorbance or percentage transmittance at a chosen wavelength. Outline why one particular wavelength has to be selected.

Deduce the equation that relates absorbance to concentration for these solutions, including the numerical value of the constant.

Estimate the absorbance of a 0.400 $\mathrm{mol\,dm^{-3}}$ solution of nickel(II) sulfate.

Predict the difference, if any, between the absorbance of the 0.400 $\mathrm{mol\,dm^{-3}}$ solution read from the extrapolated line and the value calculated from the equation in 3d.

Nickel(II) sulfate solution is green. Deduce the most suitable wavelength for this colorimetry, using section 15 of the data booklet.