Answers to Unit 3: Gases Practice Problems 1) 2.5 atm 2) a) The pressure in both cylinder is the same. Both cylinders have the same volume, temperature, and number of moles, so they must have the same pressure. According to Avogadro’s law, equal volumes of gas at the same temperature and pressure contain equal numbers of gaseous particles. b) The pressure would increase by a factor of four, because the temperature is the same and V2 = 0.25 V1. c) The pressure inside the cylinder would decrease to half of its original value, because the temperature is the same and V2 = 2 V1. 3) 276 L HF 4) 58.9 g/mol 5) 1.2 L H2(g) 6) 2.4 atm 7) a) PCH4 = 3.9 atm; PO2 = 8.1 atm; PHe = 5.7 atm b) 17.7 atm c) XCH4 = 0.22; XO2 = 0.46; XHe = 0.32 d) XO2 = 0.014; XH2O = 0.44; XCO2 = 0.22; XHe = 0.32 8) 0.183 g O2 9) a)The root mean square velocity of helium is greater than that of krypton. This is due to the fact that the molar mass of helium is less than that of krypton. The square root of the molar mass is in the denominator of the equation. A smaller denominator (molar mass) will yield a higher velocity. The temperature is the same for both gases. The molar mass is the only variable that changes. b) The temperature must increase in order to increase the average speed of the atoms in either container. 10) The 2 factors that cause deviations from ideal behavior are an increase in forces of attraction between particles and a reduction in free space relative to the volume of the container. At low temperatures, gas particles move slower and are closer together. Forces of attraction exist between gas particles when they are in close proximity to one another. These attractive forces reduce the expected pressure exerted by the gas. The ideal gas law assumes that gas particles have no volume of their own, because their volume is negligible compared to the overall volume of the container. When the volume of the particles makes up a relatively high percentage of the total volume inside the container, the gas does not behave ideally. 11) This is because the average distance between gas particles increases as the pressure decreases. When gas particles are further apart, forces of attraction between the particles are greatly reduced. This causes gas particles to behave as though they have no interactions with one another except for elastic collisions. Thus, there is no reduction in the expected pressure due to forces of attraction between particles at low pressures. 12) 176.4 g/mol 13) 63.2 g/mol