2024 TMJC Prelim H2 Bio Paper 4 (A)

1 CANDIDATE NAME CIVICS GROUP _______________________________________________________________________________ H2 BIOLOGY 9744/04 Practical Paper 22 Aug 2024 2 hours 30 minutes Candidates answer on the Question Paper. _______________________________________________________________________________ READ THESE INSTRUCTIONS FIRST Do not open this booklet until you are told to do so. Write your name, civics group and index number on all the work you hand in. Give details of the practical shift and laboratory, where appropriate, in the boxes provided. Write in dark blue or black pen. You may use an HB pencil for any diagrams and graphs. Do not use staples, paper clips, glue or correction fluid/tape. Answer all questions in the spaces provided on the Question Paper. The use of scientific calculator is expected, where appropriate. You may lose marks if you do not show your working or if you do not use appropriate units. At the end of the practice, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. This document consists of 18 printed pages and 2 blank pages. Shift Laboratory For examiner’s Use 1 / 22 2 / 16 3 / 17 Total / 55 TAMPINES MERIDIAN JUNIOR COLLEGE JC2 PRELIMINARY EXAMINATION Suggested Answers

2 Answer all questions. 1 Agar stained with universal indicator can be used to investigate diffusion. When hydrochloric acid diffuses into the agar it changes the colour from grey to pink. You will investigate the diffusion of different concentrations of hydrochloric acid in agar. You are provided with the materials shown in Table 1.1. Table 1.1 labelled contents hazard volume / cm3 H1 2.0 mol dm-3 hydrochloric acid irritant 20 W distilled water None 30 - 2 Petri dishes containing agar - - sheet T sheet T - - If H1 comes into contact with your skin, wash off immediately with cold water. It is recommended that you wear suitable eye protection. You will need to: • prepare different concentrations of hydrochloric acid, H1 − H4 • measure the diffusion distance for each concentration of hydrochloric acid and for an unknown concentration of hydrochloric acid, U. You will need to carry out a serial dilution of the 2.0 mol dm -3 hydrochloric acid, H1, to reduce the concentration by half between each successive dilution. You will need to prepare three concentrations of hydrochloric acid , H2, H3 and H4, in addition to 2.0 mol dm-3 hydrochloric acid, H1. After the serial dilution is completed, you will need to have 5 cm 3 of each concentration available to use.

3 (a) (i) Complete the table below to show how you will prepare your serial dilution. [3] Solution Concentration of hydrochloric acid / mol dm-3 Solution to be diluted Volume of solution to be diluted /cm3 Volume of water /cm3 H1 2.0 - - - H2 1.0 H1 5.0 5.0 H3 0.5 H2 5.0 5.0 H4 0.25 H3 5.0 5.0 1. Correct concentrations: 1.0, 0.5, 0.25 2. Correct solutions to be diluted + volume of these solutions = 5.0cm3 + volume of water to be added = 5.0cm3 3. Volumes expressed to 1 d.p. GENERAL COMMENTS: Always take into account the precision of instruments used for measurements! Refer to practical 1 in JC1. What is the precision of the 10cm3 syringe that was used in this procedure? ………………………………

4 1 Prepare the concentrations of hydrochloric acid, as decided in (a)(i), in the beakers provided. The different concentrations of hydrochloric acid will be put into wells cut into the agar. The position of one well is shown in Fig. 1.1. You need to decide where to put four more wells in the agar in each Petri dish so that the wells are positioned away from each other and away from the edge of the Petri dish. Hydrochloric acid will diffuse into the agar around each well. (ii) Complete Fig. 1.1 by: • drawing four small circles to show where you have decided the wells should be positioned in the agar • labelling the five small circles in Fig. 1.1 with the concentrations of hydrochloric acid you prepared in step 1 and with a sample of unknown concentration of hydrochloric acid, U. [1] • draws four circles, with one in each quadrant, and labels each with the concentration of acid (not just H1, H2, H3, H4) Fig. 1.1

5 Carry out steps 2 to 10. 2 Draw a mark on the outside edge of the Petri dishes containing agar. 3 Put the Petri dish on Fig. 1.1 so that the mark on the edge of the Petri dish lines up with the arrow in Fig. 1.1. Keep the Petri dish in this position over Fig. 1.1 for the whole of step 4. 4 Use a straw to cut wells in the agar at the positions of the small circles on Fig. 1.1: • put the end of a straw on the surface of the agar over the centre small circle • carefully push the straw into the agar • lift up the straw to leave a well in the agar • hold the straw over the container labelled For waste • squeeze the straw gently to remove the agar from the end of the straw • if the small circle of agar is not lifted by the straw, use a scalpel or mounted needle to carefully remove the agar and put it in the container labelled For waste. 5 Remove the Petri dish from Fig. 1.1 and place the Petri dish on the circle on sheet T so that the mark on the edge of the Petri dish lines up with the arrow. 6 With reference to Fig. 1.2, u se a pipette to carefully fill the appropriate well in the agar with 2.0 mol dm–3 hydrochloric acid, H1, as labelled in Fig. 1.1. Fig. 1.2 7 Repeat step 6 for the other concentrations of hydrochloric acid labelled in Fig. 1.1. Do not move the Petri dish after the wells have been filled with the hydrochloric acid. 8 Start timing. Between step 8 and step 9, you will be leaving the Petri dish on sheet T for 20 minutes. Use this time to continue with other parts of Question 1.

6 9 After leaving the Petri dish for 20 minutes, use a pipette to remove any remaining hydrochloric acid from all the wells in the Petri dish. Put this hydrochloric acid into the beaker labelle d For waste. 10 Measure the diffusion distance, as shown in Fig. 1.3, for each concentration of hydrochloric acid and U. Fig. 1.3 11 Record your results in (a)(iii). (iii) Record your results in an appropriate table. [4] Solution Concentration of hydrochloric acid / mol dm-3 Diffusion distance / mm H1 2.0 9 H2 1.0 8 H3 0.5 7 H4 0.25 6 U - 8 1. Heading for independent variable: concentration of hydrochloric acid / mol dm-3 + Heading for dependent variable: diffusion distance /mm 2. All 5 distances recorded + in whole mm / nearest 0.5mm consistently [Accept: cm] [Distance should be above 0mm, less than 11mm] 3. Correct trend with corresponding concentrations of HCl from 2.0 to 0.25 mol dm-3 4. Distance of U falls between that of 0.5 to 1.0 mol dm-3 inclusive (iv) Using your results in (a)(iii), estimate the hydrochloric acid concentration of U. [1] Correct concentration read off the results in the table [Accept: range of concentrations]

7 (v) Explain why the estimation in (a)(iv) may not be accurate and suggest how a more accurate estimation of U could be made from the results in (a)(iii). [2] • Only 4 concentrations to establish a trend. • Use at least 5 concentrations to plot a more accurate trend. • No replicates carried out. • Calculate the average of 3 replicates for greater accuracy. • Idea that the diffusion distance between two concentrations is not seen in the table. • Plot a best-fit graph to estimate U from the graph. [Reject: use narrower intervals (since the distances are too close together) • The diffusion distances around the wells are not equal / constant. • Measure 3 diffusion distances around each well and calculate the average. • The wells were filled with HCl for an unequal amount of time as the t