Sample Task Chemistry

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Hong Kong Diploma of Secondary Education Examination

Chemistry

School-based Assessment Sample Tasks

Teachers may use the sample tasks for non-profit making educational and research purposes with proper acknowledgement.

© 留 2010 Hong Kong Examinations and Assessment Authority All Rights Reserved 2010

Chemistry School-based Assessment Practical Related Tasks Volumetric analysis Teacher Notes

NSS Chemistry Curriculum Link: Time Required: 40 minutes

Topic IV Acids and Bases

Students are expected to have acquired the knowledge of topic IV and have some experience in handling apparatuses such as pipette, burette, and volumetric flask before carrying out the school-based assessment. This experiment involves two tasks: 1. Dilution of sodium carbonate solution 2. Titration of diluted sodium carbonate solution with a standard solution of hydrochloric acid Students should perform the experiment individually. In order to facilitate large class size, both the side benches and the teacher bench may have to be used. Teachers may distribute the experimental worksheet to students beforehand for reducing the stress of the students during practical assessment. In order to test students’ knowledge of detecting the end point of a titration , an unfamiliar indicator, bromothymol blue, is used. Students have to decide the colour change at the end point. The assessment is mainly based on the written worksheet. Nevertheless, the teacher can walk around to observe student performance and to give some advice. The teacher can take note whether (1) there is a filter funnel on top of the burette when the titration is carried out; (2) there is a white tile beneath the conical flask when the titration is carried out; (3) there is air space between the tap and the tip of the burette; and (4) the students perform the experiment safely.

Chemistry School-based Assessment Practical Related Tasks Volumetric analysis Assessment Criteria

Marks 10 - 9

Remarks There are at least four titration readings (including the trial). The titration readings are recorded in 2 decimal places. The titration results are accurate (i.e. within + 0.05 cm3) and the standard deviation of the titres is small (i.e. within + 0.05 cm3). Colour changes are accurately recorded. Calculations are accurate and concise. There are at least three titration readings (including the trial). The titration readings are recorded in 1 decimal place. The titration results are reasonably accurate (i.e. within + 0.15 cm3) and the standard deviation of the titres is reasonably small (i.e. within + 0.15 cm3). Colour changes are accurately recorded. Calculations are accurate and concise. There are at least two titration readings (including the trial). The titration readings are recorded. The titration results are barely accurate (i.e. within + 0.25 cm3) and the standard deviation of the titres is reasonable (i.e. within + 0.25 cm3). Colour changes are accurately recorded. Calculations are appropriate. There is at least one titration reading (including the trial). The titration readings are recorded. Colour changes are recorded. Calculations are shown.

8–6

5–3

2–1

Chemistry School-based Assessment Practical Related Tasks Volumetric analysis Student Handout Carry out volumetric analysis using the 0.2 M hydrochloric acid, sodium carbonate solution labelled as ‘solution A’, and an acid base indicator provided. Determine the concentration of solution A. Method 1. Dilute 25.00 cm3 of solution A to 250.0 cm3 with deionized water. 2. Transfer 10.00 cm3 of the diluted solution A to a conical flask and titrate it with 0.2 M hydrochloric acid using the indicator provided.

Relevant equation Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g) Titration results Trial titration Final burette reading / cm3 Initial burette reading / cm3 Volume of titrant / cm3 Mean titre = Colour change of indicator: from Calculations to cm3

Concentration of A =

Chemistry School-based...
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