Chem Gases Key

CHEMISTRY Question 2 2. Answer the following questions related to hydrocarbons. (a) Determine the empirical formula of a hydrocarbon that contains 85.7 percent carbon by mass. 1 mol C = 7.14 mol C 12.01 g C 1 mol H = 14.3 g H × = 14.2 mol H 1.008 g H nC = 85.7 g C × nH 1 point for moles of C and moles of H 7.14 mol C 14.2 mol H : 7.14 7.14 1 mol C : 1.99 mol H 1 point for ratio of moles of C to moles of H The empirical formula is CH2 1 point for correct formula (b) The density of the hydrocarbon in part (a) is 2.0 g L–1 at 50°C and 0.948 atm. (i) Calculate the molar mass of the hydrocarbon. mass RT molar mass mass RT RT = density × molar mass = × V P P L atm 0.0821 × 323 K mol K molar mass = 2.0 g L−1 × 0.948 atm −1 molar mass = 56 g mol PV = nRT = 1 point for correct substitution 1 point for the answer (ii) Determine the molecular formula of the hydrocarbon. empirical mass × n = molar mass empirical mass for CH2 = 14 g mol−1 14 g mol−1 × n = 56 g mol−1 n = 4 The molecular formula is C4H8. 1 point for correct formula CHEMISTRY Question 2 (cont’d.) (c) Two flasks are connected by a stopcock as shown below. The 5.0 L flask contains CH4 at a pressure of 3.0 atm, and the 1.0 L flask contains C2H6 at a pressure of 0.55 atm. Calculate the total pressure of the system after the stopcock is opened. Assume that the temperature remains constant. Pf of CH4 = Pf of C2H6 = PV (3.0 atm) (5.0 L) i i = = 2.5 atm CH4 Vf 6.0 L 1 point for final pressure of CH4 or C2H6 PV (0.55 atm) (1.0 L) i i = = 0.092 atm C2H6 Vf 6.0 L PT = Pf CH4 + Pf C2H6 = 2.5 atm + 0.092 atm = 2.6 atm 1 point for the total pressure (d) Octane, C8H18(l), has a density of 0.703 g mL–1 at 20°C. A 255 mL sample of C8H18(l) measured at 20°C reacts completely with excess oxygen as represented by the equation below. 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g) Calculate the total number of moles of gaseous products formed. n products = 255 mL C8H18 × 0.703 g C8 H18 1 mol C8 H18 × × 1 mL C8 H18 114 g C8 H18 34 mol products = 26.7 mol products 2 mol C8 H18 1 point for substitution of any of these conversion factors 1 point for the correct answer