Gases Practice Problem 2004D Judy Hugh

2004D Question Answer the following questions about carbon monoxide, CO(g), and carbon dioxide, CO 2 (g). Assume that both gases exhibit ideal behavior.  (a) Draw the complete Lewis structure for the CO molecule and for the CO 2 molecule.  (b) Identify the shape of the CO 2 molecule.  (c) One of the two gases dissolves readily in water to form a solution with a pH below 7. Identify the gas and account for this observation by writing a chemical equation.  (d) A 1.0mol sample of CO(g) is heated at constant pressure. On the graph below, sketch the expected plot of volume versus temperature as the gas is heated. (question continues…)

2004D Question cont’d  (e) Samples of CO(g) and CO 2 (g) are placed in 1L containers at the conditions in the diagram below i) Indicate whether the average kinetic energy of the CO 2 is greater than, equal to, or less than the average kinetic energy of the CO(g) molecules. Justify your answer. ii) Indicate whether the root-mean-square speed of the CO 2 (g) molecules is greater than, equal to or less than the root-mean- square speed of the CO(g) molecules. Justify your answer. iii) Indicate whether the number of CO 2 (g) molecules is greater than, equal to, or less than the number of CO(g) molecules. Justify your answer.

Part a) Answer CO: one triple bond sharing 6 electrons, and a pair of electrons on sides of both Carbon and Oxygen CO 2 : two double bonds (from Carbon to each of the two Oxygens), and 2 pairs of electrons on each Oxygen (total of 4 electrons)

Part b) Answer CO 2 has linear molecular geometry AB 2 geometry  1 central atom (AB 2 )  2 bonded atoms (AB 2 )  0 lone pairs (no E)

Part c) Answer CO 2 is the gas that will form an acidic solution with a pH less than 7 with the chemical equation: CO 2 (g) + H 2 O (l)  HCO 3 - (aq) + H + (aq) You know that it is CO 2 because it forms H + in the product side, making it an acid. CO + H 2 O would not form an H +

Part d) Answer If a 1.0 mol sample of CO (g) were heated at constant pressure, the volume vs. temperature graph would look like this because (click!)because

Part d) Answer Explanation According to Charles’ Law V 1 /T 1 = V 2 /T 2 at constant pressure, Volume and Temperature are directly proportional –– as the temperature increases, the volume increases at the same rate, creating a straight line with a positive slope of 1 (as shown in the graph)

Part e) Answer i) The Average Kinetic Energies are the same for both samples because Average Kinetic Energy is dependent on temperature  Therefore if the temperature is the same throughout, the average kinetic energies are equal ii) The root-mean-square speed for CO 2 is lower than the root-mean-square speed for CO because the molar mass of CO 2 (44 g/mol) is higher than that of CO (28 g/mol), and the root-mean-square speed is inversely proportional to the square root of the molar mass of the gas  Therefore the heavier the molar mass, the lower the root- mean-square speed (which was the case for CO 2 )

Part e) Answer cont’d iii) There are fewer CO 2 molecules than CO molecules because of Avogadro’s Hypothesis, which states that “ Equal volumes of gas at the same temperature and pressure contain equal number of molecules.”  In the diagram shown, CO 2 (1atm) has half the pressure of CO (2atm), therefore it must have half the number of molecules

FINISHED!