2020年11月IB化学SL真题下载-Paper3
1. In order to determine the oil content of different types of potato crisps (chips), a student weighed 5.00 g of crushed crisps and mixed them with 20.0 cm3 of non-polar solvent.
She assumed all the oil in the crisps dissolved in the solvent.
The student then filtered the mixture to remove any solids, and gently heated the solution on a hot plate to evaporate the solvent.
She measured the mass of the oil that remained from each type of crisps.
(a) Suggest why a non-polar solvent was needed.
(b) State one reason why the mixture was not heated strongly.
(c) Non-polar solvents can be toxic. Suggest a modification to the experiment which allows the evaporated solvent to be collected.
(d) Suggest one source of error in the experiment, excluding faulty apparatus and human error, that would lead to the following:
Experimental mass greater than actual mass of oil in crisps:
Experimental mass lower than actual mass of oil in crisps:
2. An investigation was carried out to determine the effect of chain length of the alcohol on the equilibrium constant, Kc, for the reversible reaction:
The reactants, products and the catalyst form a homogeneous mixture.
Fixed volumes of each alcohol, the ethanoic acid and the sulfuric acid catalyst were placed in sealed conical flasks.
At equilibrium, the flasks were placed in an ice bath, and samples of each flask titrated with
NaOH (aq) to determine the ethanoic acid concentration present in the equilibrium mixture.
The following processed results were obtained.
(a) Identify the independent and dependent variables in this experiment.
Independent variable:
Dependent variable:
(b) The ice bath is used at equilibrium to slow down the forward and reverse reactions.Explain why adding a large amount of water to the reaction mixture would also slow down both reactions.
(c) Suggest why the titration must be conducted quickly even though a low temperature is maintained.
(d) An additional experiment was conducted in which only the sulfuric acid catalyst was titrated with NaOH (aq). Outline why this experiment was necessary.
(e) Calculate the percentage uncertainty and percentage error in the experimentally determined value of Kc for methanol.
Percentage uncertainty:
Percentage error:
(f) Comment on the magnitudes of random and systematic errors in this experiment using the answers in (e).
(g) Suggest a risk of using sulfuric acid as the catalyst.
2020年11月IB化学SL真题余下省略！
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