2017年5月IB化学SL真题下载-Paper3
A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.
Volume of aqueous NaOH = 25.0 ± 0.5 cm3
Concentration of HCl = 1.00 ± 0.01 mol dm−3
1. (a) Calculate the percentage uncertainty of the volume of the aqueous sodium hydroxide.
(b) Suggest how the precision of this measurement could be improved.
2. (a) Deduce why more heat was produced in mixture B than in mixture A.
(b) Deduce why the temperature is higher in mixture C than in mixture D.
3. Suggest how the end point of the titration might be estimated from the graph.
4. State and explain how the graph would differ if 1 mol dm−3 sulfuric acid had been used instead of 1 mol dm−3 hydrochloric acid.
5. The graph of temperature against titre can be used to calculate the concentration of alkali without knowing the concentration of the hydrochloric acid, using the enthalpy of neutralization.
(a) Explain how the concentration may be calculated in this way.
(b) Heat losses would make this method less accurate than the pH probe method. Outline why the thermometric method would always give a lower, not a higher, concentration.
(c) Suggest how heat loss could be reduced.
(d) State one other assumption that is usually made in the calculation of the heat produced.
(e) Suggest why scientists often make assumptions that do not correspond to reality.
(f) Outline why the thermochemical method would not be appropriate for 0.001 mol dm−3 hydrochloric acid and aqueous sodium hydroxide of a similar concentration.
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