Salicylic acid acts as a painkiller and is the active ingredient in aspirin. When a person ingests a dose of aspirin, the salicylic acid enters the blood and circulates in the bloodstream. Most of the salicylic acid is metabolised by the body. Some of the salicylic acid is excreted by the kidneys into the urine. You will estimate the concentration of salicylic acid in two solutions, S1 and S2. These solutions represent samples of blood and urine taken from a person who has ingested aspirin. Note: you will not be working with real blood or real urine. You are provided with the materials shown in Table 1.1. If any solution comes into contact with your skin, wash off immediately with cold water. It is recommended that you wear suitable eye protection. The concentration of salicylic acid can be determined by using iron(III) chloride, C, which forms a purple solution when mixed with salicylic acid. The greater the concentration of salicylic acid, the more intense the purple colour formed. You will need to: - prepare different concentrations of salicylic acid - record the intensity of purple colour for each concentration - $$\(\quad\)$$ estimate the concentration of salicylic acid in S1 and S2. You will use proportional dilution to make different concentrations of salicylic acid. You will prepare $$\(10 \mathrm{~cm}^{3}\)$$ of each concentration, using $$\(\mathbf{A}\)$$ and $$\(\mathbf{W}\)$$. Table 1.2 shows how to prepare two of the concentrations you will use. Decide which other concentrations of salicylic acid you will use. Complete Table 1.2 to show how you will prepare the concentrations of salicylic acid you will use. Carry out step 1 to step 7. step 1 In the beakers provided, prepare the concentrations of salicylic acid, as shown in Table 1.2. step 2 Label the test-tubes with the concentrations of salicylic acid prepared in step 1. step 3 Put $$\(2 \mathrm{~cm}^{3}\)$$ of $$\(\mathbf{C}\)$$ into each of the test-tubes labelled in step 2. step 4 Put $$\(5 \mathrm{~cm}^{3}\)$$ of $$\(\mathbf{A}\)$$ into the test-tube labelled 1.0\%. Use a glass rod to mix. step 5 Repeat step 4 for each of the other concentrations you prepared in step 1. step 6 Place the white card behind the test-tubes and observe the intensity of colour in each test-tube. You may see the same intensity in more than one test-tube. step 7 Compare the intensity of colour in each test-tube with the key in Fig. 1.1. Record your observations in (a)(ii) using only the symbols shown in the key in Fig. 1.1. (ii) Record your results in an appropriate table. Carry out step 8 to step 12. step 8 Label one test-tube $$\(\mathbf{S 1}\)$$ and label another test-tube $$\(\mathbf{S 2}\)$$. step 9 Put $$\(2 \mathrm{~cm}^{3}\)$$ of $$\(\mathbf{C}\)$$ into each of the test-tubes labelled in step 8. step 10 Put $$\(5 \mathrm{~cm}^{3}\)$$ of $$\(\mathbf{S} \mathbf{1}\)$$ into the appropriately labelled test-tube. Use a glass rod to mix. step 11 Put $$\(5 \mathrm{~cm}^{3}\)$$ of $$\(\mathbf{S} \mathbf{2}\)$$ into the appropriately labelled test-tube. Use a glass rod to mix. step 12 Observe the intensity of colour in each test-tube. (iii) Record your observations for $$\(\mathbf{S} 1\)$$ and $$\(\mathbf{S} 2\)$$ using the symbols shown in the key in Fig. 1.1. intensity of colour for $$\(\mathbf{S 1}\)$$ ........................................................... .. intensity of colour for $$\(\mathbf{S} \mathbf{2}\)$$ ........................................................... (iv) Use your results in (a)(ii) and (a)(iii) to estimate the concentration of salicylic acid in S1 and S2. ........................................................ ..% ........................................................ (v) When a person ingests a dose of aspirin, some of the salicylic acid is excreted by the kidneys into the urine. State which sample, $$\(\mathbf{S 1}\)$$ or $$\(\mathbf{S 2}\)$$, is from the person's blood. Explain your answer. sample explanation .................................................................................................................... ....................................................................................................................................... . ....................................................................................................................................... . (vi) State the independent variable in this investigation. ................................................................................................................................. (vii) Describe one significant source of error when carrying out step 6 and step 7 and suggest an improvement to reduce this error. source of error ............................................................................................................... improvement ................................................................................................................. ....................................................................................................................................... .

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