Dalton ’ s Law of Partial Pressures Learning Goals … … use Dalton’s Law to find the partial pressure of a gas... use Dalton’s Law of Partial pressures to find the pressure of a dry gas when collected over water

When you have a mixture of gases, you can determine the pressure exerted by each gas separately. This is called the partial pressure of each gas Since each gas has the same power to cause pressure, the partial pressure of a gas depends on how much of the mixture is composed of each gas (in moles). Dalton ’ s Law of Partial Pressures 1 L oxygen 50 kPa 1 L nitrogen 100 kPa 1 L mixed gas 150 kPa +=

Consider air, a mixture of mostly nitrogen and oxygen gases. Also, P total = P O 2 + P N 2 This idea is used when a gas is collected over water. P atm = P gas + P H 2 O P H 2 O is found on a chart moles O 2 = P O 2 moles total P total moles N 2 = P N 2 moles total P total

Collecting Gases Over Water Many times gases are collected over H 2 O Often we want to know the volume of dry gas at STP (useful for stoichiometry). For this we must make 3 corrections: 1.The level of water inside and outside the tube must be level (so pressure inside is equal to the pressure outside). 2.The water vapour pressure must be subtracted from the total pressure (to get the pressure of the dry gas). 3.Finally, values are converted to STP using the combined gas law.

Ex 1)A gas was collected over 21°C H 2 O at kPa. After equalizing water levels, the volume was 325 mL. Give the volume of dry gas at STP. (P 1 )(V 1 )(T 2 ) (P 2 )(T 1 ) V2=V2= (100.4 kPa)(325 mL)(273 K) (101.3 kPa)(294 K) = = 299 mL V 1 = 325 mL T 1 = 294 K P 1 = kPa = kPa T 2 = 273 K V 2 = ? P 2 = kPa kPa  The volume of the gas at STP is 299 mL. At 21°C the water vapour pressure is 2.5 kPa.

V 1 = 85.0 mL T 1 = 298 K P 1 = 120 kPa = kPa T 2 = 328 K V 2 = ? P 2 = 130 kPa V 2 = P 1 V 1 T 2 P 2 T 1 V 2 = (116.8kPa)(85.0mL)(328K) (130kPa)(298K) V 2 = 84.1 mL At 25°C the water vapour pressure is 3.2 kPa kPa Ex 2)During a lab, a student collects 85.0 mL of gas over water at 25°C and 120 kPa. What volume would the dry gas occupy at 55°C and 130kPa?  The volume of the gas would be 84.1 mL.

CAN I … … use Dalton’s Law to find the partial pressure of a gas... use Dalton’s Law of Partial pressures to find the pressure of a dry gas when collected over water Homework WS “Dalton’s Law of Partial Pressures”