Question 124 marks
Question
You are going to investigate osmosis. You will be using a model cell made from dialysis tubing. Dialysis tubing is permeable to water but not permeable to larger molecules such as sucrose, which is a type of sugar. Read all the instructions but DO NOT CARRY THEM OUT until you have drawn a table for your results in the space provided in 1(a)(i). You should use the safety equipment provided while you are carrying out the practical work. Step 1 Label one test-tube DW and the other S. Place both test-tubes in the test-tube rack. Step 2 Fill the syringe with 10 cm3 of sucrose solution. Step 3 Remove one knotted piece of dialysis tubing from the container. Open the unknotted end by rubbing it gently between your fingers. Carefully transfer the 10 cm3 of sucrose solution from the syringe into the dialysis tubing bag. Step 4 Hold the top of the dialysis tubing bag containing the sucrose solution as shown in Fig. 1.1. Use the ruler to measure the distance from the knot to the meniscus of the liquid in the bag. Think about how you will ensure that this measurement can be made consistently. Record this measurement in your table in 1(a)(i). finger dialysis tubing bag measure from the knot to bottom of the meniscus the meniscus knot Fig. 1.1 Step 5 Place the dialysis tubing bag into the test-tube labelled S. Fold the open end of the bag over the top of the test-tube and secure with an elastic band, as shown in Fig. 1.2. elastic band open end of the dialysis tubing bag test-tube Fig. 1.2 Step 6 Fill the syringe with 10 cm3 of distilled water. Step 7 Remove the second knotted piece of dialysis tubing from the container. Open the unknotted end by rubbing it gently between your fingers. Carefully transfer the 10 cm3 of distilled water from the syringe into the dialysis tubing bag. Step 8 Repeat steps 4 and 5 with the dialysis tubing bag containing the distilled water and the test-tube labelled DW. Record the measurement for distilled water in your table in 1(a)(i). Step 9 Use the measuring cylinder to measure 30 cm3 of distilled water. Pour the distilled water into test-tube S, as shown in Fig. 1.3. elastic band open end of the dialysis tubing bag test-tube 30 cm3 of distilled water Fig. 1.3 Step 10 Repeat step 9 for test-tube DW. Step 11 Place test-tubes S and DW into the empty 250 cm3 beaker. Raise your hand when you are ready for 200 cm3 of hot water to be added to the beaker to make a hot water-bath. Step 12 Start the stop-clock and leave the test-tubes in the hot water-bath for 15 minutes. Continue with the other questions while you are waiting. Step 13 After 15 minutes remove test-tubes S and DW from the hot water-bath and place them in the test-tube rack. Step 14 Carefully remove the dialysis tubing bag from test-tube S. Hold the top of the bag and use the ruler to measure the distance from the knot to the meniscus of the liquid in the bag. Ensure that your measurement starts from the same place as the measurement made in step 4. Record this measurement in your table in 1(a)(i). Step 15 Repeat step 14 with test-tube DW. (a) (i) Prepare a table to record your results. [4] (ii) Calculate the change in distance from the knot to the meniscus of the solution in the dialysis tubing bag in test-tubes S and DW. S …… mm DW …… mm [1] (iii) Explain why it was important to take the measurements in step 4 and step 14 from the same place on the dialysis tubing. …… [1] (iv) State a conclusion for the results of this investigation. …… [1] (v) Identify the variable that was measured (dependent variable) in this investigation. …… [1] (b) (i) One syringe was used for both step 2 and step 6. This is a source of error. Explain why this was a source of error and how this could have affected the results. error …… effect on the results …… [2] (ii) Only one set of results was collected during this investigation. Explain why it is better to collect several sets of results. …… [1] (c) Students investigated the effect of sucrose concentration on the mass of potato cylinders. • A potato was cut into cylinders. • The potato cylinders were all cut to 2 cm in length. • The initial mass of each potato cylinder was measured and recorded. • Each potato cylinder was put into a different concentration of sucrose solution. • The potato cylinders were left in the sucrose solutions for one hour. • The potato cylinders were removed from the sucrose solutions and the final mass of each potato cylinder was measured and recorded. (i) State two variables that were kept constant in this investigation. 1 …… 2 …… [2] The results of the investigation are shown in Table 1.1. Table 1.1 concentration initial mass of final mass of percentage change in of sucrose potato cylinder potato cylinder change in mass / g / mol per dm3 /g /g mass 0.00 2.13 2.29 0.16 7.5 0.20 2.05 2.08 0.03 1.5 0.40 2.52 2.42 –0.10 –4.0 0.60 1.68 1.52 –0.16 –9.5 0.80 1.56 1.32 –0.24 –15.4 1.00 2.51 2.08 –0.43 (ii) Calculate the percentage change in mass of the potato cylinder that was immersed in 1.00 mol per dm3 sucrose solution. Give your answer to one decimal place. Space for working. ……% [3] (iii) Plot a line graph on the grid of the concentration of sucrose solution against the percentage change in mass. One axis has been started for you. Include a curved line of best fit on your graph. 0.0 [4] (iv) Estimate the concentration of sucrose solution at which there was no percentage change in the mass of the potato cylinder. …… mol per dm3 [1] (v) Explain why the percentage change in mass is more useful than the change in mass when analysing the results in Table 1.1. …… [1] (d) State the name of the solution that is used to test for the presence of starch and give the result of a positive test. test solution …… positive result …… [2] [Total: 24]
Mark scheme
1(a)(i) table drawn with minimum three columns and a header line ; 4 appropriate column / row headings, with units ; four suitable measurements ; correct trend ; 1(a)(ii) S = correct calculation from candidate’s results (mm) 1 and DW = 0 (mm) ; 1(a)(iii) to allow comparison / to calculate the difference accurately / AW ; 1 1(a)(iv) suitable conclusion based on the candidate’s results ; 1 1(a)(v) distance (from the meniscus to the knot) / AW ; 1 1(b)(i) error: 2 idea of contamination ; effect: the distilled water distance is greater than expected / there is less difference between S and DW / AW ; 1(b)(ii) to identify anomalous results / to calculate an average ; 1 1(c)(i) (initial) length / surface area (of cylinders) ; 2 (soaking) time ; type of potato / same potato ; 1(c)(ii) (-)17.1(%) ;;; 3 MP1 for evidence of dividing by 2.51 MP2 for correct percentage MP3 for correct rounding to one decimal place 1(c)(iii) axes labelled with units ; 4 linear scale and occupies at least half the grid ; first five points plotted accurately ± half a small square ; suitable (curved) line of best fit drawn ; 1(c)(iv) the sucrose concentration value at which the candidate’s graph 1 line intersects at 0.0 percentage change in mass ± half small square ; 1(c)(v) (to allow comparison as) the initial masses were different ; 1 1(d) iodine (solution) ; 2 blue–black ;