Brass is an alloy of copper and zinc. An experiment is completed to find the percentage by mass of copper in a sample of brass. In the experiment, the sample of brass is reacted with an excess of concentrated nitric acid, HNO3. This forms a solution containing Cu2+(aq) ions. The amount of Cu2+(aq) ions formed is determined by titration. A student uses the following method. step 1 Weigh a glass beaker and record the mass. step 2 Add approximately 1.00g of powdered brass to the beaker and record the mass of the beaker and the brass. step 3 Transfer the brass into conical flask A which contains excess concentrated HNO3. step 4 Reweigh the glass beaker and record the mass. step 5 Add aqueous sodium carbonate, Na2CO3(aq), dropwise to flask A until a precipitate of copper(II) carbonate, CuCO3(s), appears. Then add dilute ethanoic acid dropwise until the precipitate is fully dissolved. step 6 Transfer all the contents of flask A to a 100.0cm³ volumetric flask and make up to the mark with distilled water. This is solution B. step 7 Transfer 10.0 cm³ of solution B into conical flask C. step 8 Add 10cm³, an excess, of aqueous potassium iodide, KI(aq), to flask C. step 9 Titrate the contents of flask C against 0.0600moldm⁻³ sodium thiosulfate, Na2S2O3(aq), using starch solution as an indicator. step 10 Repeat steps 7 to 9 until concordant results are obtained.

1. (a)Both copper and zinc in brass react with concentrated HNO3 in step 3. reaction 1 Zn(s) + 4HNO3(aq) → Zn(NO3)2(aq) + 2NO2(g) + 2H2O(I) reaction 2 Cu(s) + 4HNO3(aq) → Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l) Suggest why step 3 in the experiment should be completed in a fume cupboard.[1]
2. (b)Explain why the glass beaker is reweighed in step 4.[1]
3. (c)Name a suitable piece of apparatus to transfer the 10.0 cm³ of solution B in step 7.[1]
4. (d)Identify the substance used to rinse the burette before step 9 is done for the first time.[1]
5. (e)In step 9, 0.0600 mol dm⁻³ Na2S2O3(aq) is used. This solution is prepared from 0.200 mol dm⁻³ Na2S2O3(aq) before the experiment. Describe how to make a 100.0cm³ standard solution of 0.0600 moldm⁻³ Na2S2O3(aq) from the 0.200 moldm⁻³ Na2S2O3(aq) solution. Give the name and capacity of any apparatus you would use. Write your answer in a series of numbered steps.[3]
6. (f)The measurements collected during steps 1 to 4 are shown in Table 1.1. Table 1.1 mass of glass beaker/g | 25.55 mass of glass beaker containing powdered brass/g | 26.65 mass of glass beaker after transferring brass to conical flask A/g | 25.65 Determine the mass of powdered brass added to conical flask A. mass of powdered brass =[1]
7. (g)(i)The volumes measured in each of the titrations are shown in Table 1.2. Table 1.2 rough titration | titration 1 | titration 2 | titration 3 final burette reading/cm³ | 24.05 | 24.80 | 45.35 | 22.50 initial burette reading/cm³ | 3.25 | 4.50 | 24.80 | 2.10 titre/cm³ Complete Table 1.2.[1]
8. (g)(ii)Calculate a suitable mean titre to use in the calculations. mean titre =[1]
9. (g)(iii)Calculate the percentage error in the titre in titration 3. Show your working. percentage error =[1]
10. (h)(i)Using a second sample of brass, another student determined the mean titre to be 17.35 cm³ of 0.0600 mol dm⁻³ Na2S2O3(aq). Calculate the amount, in mol, of Na2S2O3 in this student's mean titre. amount of Na2S2O3 =[1]
11. (h)(ii)Use the equation for reaction 4 and your answer to (h)(i) to determine the amount, in mol, of I2 that reacted with the Na2S2O3. amount of I2 =[1]
12. (h)(iii)Use the equation for reaction 3 and your answer to (h)(ii) to determine the amount, in mol, of Cu2+ in 10.0 cm³ of their solution B. amount of Cu2+ =[1]
13. (h)(iv)Calculate the mass of copper present in the second sample of powdered brass. mass of copper =[1]
14. (h)(v)The mass of the second sample of powdered brass was 1.05g. Calculate the percentage by mass of copper in the second sample of powdered brass. Give your answer to three significant figures. percentage by mass of copper =[1]