MASTERING THE VCE 2014 UNIT 3 CHEMISTRY STUDENT SOLUTIONS

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1 MASTERING THE VCE 2014 UNIT 3 CHEMISTRY STUDENT SOLUTIONS FOR ERRORS AND UPDATES, PLEASE VISIT QUESTION 45 QUESTION 46 Answer is A QUESTION 47 The number of protons in the element. The number of protons in an atom within a specific period of the Periodic table determines the atomic radius, and hence the strength of the attractive forces between the nucleus and the valence electrons. This in turn determines the amount of energy that is required for electrons to move to the excited state, and hence the energies that are released when these electrons return to the ground state. QUESTION 48 The energy levels are different in atoms and their corresponding ions due to the different numbers of electrons and hence different radii. This means that there will be differences in the possible energy transitions. QUESTION 49 Yes QUESTION 67 QUESTION 76 Metal chlorides are highly volatile. QUESTION 77 It is the excitation of valence electrons in ground state metal atoms that are measured in AAS. Note: The energy levels of atoms are different to their corresponding ions. QUESTION 78 The only function of the flame in absorption measurements is to convert the sample into an atomic vapour. The School For Excellence 2013 Mastering the VCE Unit 3 Chemistry Page 1

2 QUESTION 79 To supply the energy required to dissociate compounds with stronger intraparticle bonds into atoms. QUESTION 80 Cu Cu 2 ( aq) ( g ) Cl. Note - The heat of the flame assists in the oxidation process. Either air or nitrous oxide are used as oxidants. QUESTION 81 QUESTION 82 The copper and barium ions would not interfere with the measurement as the analysis would be performed using a wavelength of light that is exclusively absorbed by lead. QUESTION 83 As a solution. QUESTION 84 In the gaseous phase. QUESTION 85 QUESTION 86 QUESTION 87 Answer is C Answer is A QUESTION 88 Read the sample value off the graph = 2.6 ppm. There are 2.6 ppm of iron in the 50.0 ml diluted sample. As the original sample was diluted by a factor of 10, the concentration of the initial solution would be 10 times greater i.e. 26 ppm. In 5 ml: Concentration is 26 ppm or 6 26g per 10 g 6 26g per 10 ml (assuming 1g = 1ml) g per ml g per 1 ml 6 3 In 250 ml, there will be g of Fe or g. (d) 36% of the daily requirement. The School For Excellence 2013 Mastering the VCE Unit 3 Chemistry Page 2

3 (e) (f) The machine may also not have been calibrated correctly. Presence of particles that may scatter light. Often, there is a small amount of iron or other metal ions in water which would register in the 0.00 ppm sample. If the undiluted sample was used, the absorbance reading would not have fallen on the calibration curve. You cannot extrapolate these types of curves to obtain data. QUESTION 89 Standard curve showing the concentration of chromium versus absorbance absorbance ~0.3 ppb concentration of chromium (ppb) Concentration = mg L -1 No the water is not contaminated g of chromium The School For Excellence 2013 Mastering the VCE Unit 3 Chemistry Page 3

4 QUESTION 90 c( Na ) in diluted sauce mg / L c( Na ) in original sauce mg / L (i) NaCl was the only source of Na ions (there are no other sodium containing substances in the sample). (ii) (iii) % Daily % 2.5 (d) The light source used in AAS emits wavelengths of radiation unique to Na i.e. Therefore, the selected wavelength will only be absorbed by Na. Na (e) If the slit was removed, other sources of light (eg. emissions) would reach the detector. The amount of light reaching the detector would be higher, and this in turn would be interpreted as being due to a lower concentration of metal species in the sample. Calculated concentrations will be lower. QUESTION 91 This would otherwise result in a lower absorbance reading, and hence a lower than expected concentration. QUESTION 92 The School For Excellence 2013 Mastering the VCE Unit 3 Chemistry Page 4

5 QUESTION ppm (i) Concentrations corresponding to 2, 5 and 10 ml are 2.064, 5.16 and ppm respectively. (ii) 4.20 ppm Because the sample is decomposed into atoms in the flame, AAS measures the total concentration of the element in the sample; in this case it measures Fe(II) plus Fe(III). (d) (i) In the gaseous state. (ii) The monochromator filters out unwanted wavelengths produced during the procedure so that the detector only detects the selected radiation. (iii) The cathode lamp selected is made from the metal being analysed. The lamp will then produces the exact wavelength of light required by the atoms in the sample for excitation. The School For Excellence 2013 Mastering the VCE Unit 3 Chemistry Page 5