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School of Chemistry and Physics, Howard College Campus University of KwaZulu-Natal CHEMICAL ENGINEERING CHEMISTRY 1 (CHEM161) Class Test 2: APRIL 2014 Duration: 40 minutes Marks: 25 Student Number Student Name Tutorial Venue Tutor Question 1 (a) Calculate the frequency (in s-1) of the electromagnetic radiation emitted by a hydrogen atom when its electron undergoes a transition from the n = 4 level to the n = 2 level. (2) (b) Consider the following diagram: E3 E2 E E1 (i) Give the type of electromagnetic radiation shown in the diagram above. (½) (ii) How many lines would you expect in the spectrum for the above transitions? (½) (iii) Draw a fully labeled diagram showing the spectral lines. (1½) Question 2 (a) Write two Lewis structures for cyanamide, NH2CN. Choose the more plausible structure (Use C as the central atom). (4) 2 (b) Explain, in detail, whether you expect carbon tetrachloride (CCl4) to be polar or nonpolar (Hint. Your answer must include Lewis structures, molecular geometry and net dipole moment). (4) Question 3 (a) For the reaction C6H14(l) + O2(g) CO2(g) + H2O(l) ΔHcombustion = -3246 J mol-1 Calculate the enthalpy of formation of n-hexane (C6H14) given ΔHfo of CO2 and water are -393.5 and -285.8 kJ mol-1 respectively. (2) 3 (b) Use Hess’s law to calculate the standard enthalpy of formation of carbon disulfide (CS2) from its elements, given that C(graphite) + O2(g) CO2(g) ΔHrxno = - 393.5 kJ mol-1 S(rhombic) + O2(g) SO2(g) ΔHrxno = - 296.4 kJ mol-1 CS2(l) + 3O2(g) CO2(g) + 2SO2(g) ΔHrxno = - 393.5 kJ mol-1 (3½) Question 4 (a) The density of an unknown gas, at STP, is 1.429 g L-1. Calculate the molar mass of the gas (R = 0.0821 atm L mol-1 K-1). (2) 4 (b) In an experiment to determine the atomic weight of Al, 1.349 of Al metal was allowed to react with excess dilute H2SO4 and 1.910 L of H2 gas was evolved and collected over water at 23°C and 746 mm of Hg. Calculate the atomic weight of Al. (Vapour pressure of H2O at 23°C = 21.0 mmHg) Al(s) + H2SO4(aq) Al2(SO4)3(aq) + H2(g) (unbalanced) 5 (5)