NJC 2023 Gases Tutorial Suggested Answers

National Junior College SH1 H2 Chemistry 1 The Gaseous State Discussion Questions 1 Which option has the largest volume at 25 °C and 101 kPa? A 9 g of CH4 B 12 g of H2O C 15 g of C2H6 D 20 g of CO2 At 25 °C, H2O is in liquid state and occupy smaller volume than gas Amount of CH4 = 0.563 mol Amount of C2H6 = 0.5 mol Amount of CO2 = 0.455 mol From the Ideal Gas Equation: pV = nRT, we have V ∝ n. Hence CH4 would have the largest volume 2 2.5 dm3 of methane, CH4, at a pressure of 1.50 atm and 4.5 dm 3 of ethane, C 2H6, at a pressure of 1.00 atm were introduced into a 3.0 dm3 vessel at constant temperature. What is the final pressure of the gas mixture? A 0.825 atm B 2.50 atm C 2.75 atm D 3.57 atm For methane: p1 V1 = p2 V2 (1.50)(2.5) = (pmethane)(3.0) pmethane =1.25 atm For ethane: p1 V1 = p2 V2 (1.0)(4.5) = (pethane)(3.0) pethane =1.5 atm ptotal = pmethane + pethane = 1.25 + 1.5 = 2.75 atm 3 Which graph is not a correct description of the behaviour of a fixed mass of an ideal gas? (Note: T is measured in Kelvin scale.) A B C D From the Ideal Gas Equation: pV = nRT, we have pV ∝ T (when n is constant). With increasing T, the value of pV should increase proportionally. Hence graph B and D are correct. So the value of pV remains constant with constant T, hence a horizontal line for graph A is correct. This also means a horizontal line for graph C should be expected instead. pV 1/p 0 constant T pV T 0 constant V pV V 0 constant T pV T 0 constant p

National Junior College SH1 H2 Chemistry 2 4 A vessel contained a gas mixture of nitrogen and oxygen at 30 C and a pressure of 200 kPa. After all the oxygen has reacted with zinc, the total pressure in the vessel was 150 kPa at 30 C. 2 Zn (s) + O2 (g) → 2 ZnO (s) What was the molar ratio of nitrogen to oxygen in the mixture initially? A 1 : 3 B 2 : 3 C 3 : 1 D 3 : 2 After the reaction has occurred, only N2 is left in the vessel. So pnitrogen = 150 kPa ptotal = pnitrogen + poxygen = 200 kPa So poxygen = 50 kPa Since p ∝  at constant temperature , nnitrogen noxygen = pnitrogen poxygen = 150 50 = 3 1 5 A container contains 1 dm3 of gas X and 1 dm3 of gas Y at s.t.p. Both X and Y behave as ideal gases and the relative molecular mass of X is 4 times that of Y. Which statement is true? A The amount of gas X is 4 times that of Y. B The partial pressure of each gas is 50 kPa. C The total pressure of the system will decrease when 1 mole of Ne is pumped into the container. D The volume of the container doubles when the temperature of the system increases from 20 C to 40