48 Argon is a noble gas that is used for welding. It is sold in 14 L gas cylinders, like the one in the photo, at a pressure of 200 bar when the temperature is 27 ºC. It is used until no more gas comes out, at which point the pressure inside the cylinder equals that of the atmosphere. Data: 1 bar = 1.013 atm a) What is the mass of the argon in the full gas cylinder? b) What mass of argon remains inside the cylinder when no more gas comes out? 49 The graph below shows the relationship between the pressure and the volume of a gas at 50 ºC. p (atm) 1.5 2 3 6 12 V (L) 16 12 8 4 2 a) What type of relationship is there between both quantities? Write the mathematical equation that represents it. b) What volume will the gas occupy if it reaches a pressure of 2 500 hPa at 50 ºC? c) What volume will it occupy if the pressure reaches 1 500 hPa at 100 ºC? 50 Cartridges of CO2 gas are sold to inflate bicycle tyres. One of them contains 16 g of gas in a volume of 30 mL. a) What will be the pressure inside the cartridge on a day when the temperature is 27 ºC? b) What mass of nitrogen gas (N2) should be put into that same cartridge to exert the same pressure at the same temperature as the CO2 gas is at? 51 Taking into account the kinetic theory of gases, explain why no substance can be at a temperature below -273.15 ºC. Is there also a maximum value for the temperature a substance can be at? 52 P H Y S I C S A N D S H O W S At some events a CO2 smoke machine is used to create special effects. This machine can fill a space with a kind of fog that disappears after a few seconds. There are several models of CO2 smoke machines. In the most common type, the gas comes out at a very high pressure and expands. This produces a 6 or 7 m column. a) Initially, the CO2 was in high-pressure gas cylinders. What volume does the gas occupy in a concert hall? What happens to the pressure of the gas in the process? b) Use the kinetic theory of gases to explain the journey of the gas from when it leaves the machine to when it becomes a kind of fog. Solutions 53 Answer in your notebook. In 100 g of a 10 % solution by mass, there are: a) 10 g of solute and 100 g of solvent. b) 20 g of solute and 80 g of solvent. c) 10 g of solute and 90 g of solvent. 54 Vinegar is a solution of acetic acid in water at 3% by mass. Calculate: a) Which is the solute and which is the solvent. b) The amount of solute in 50 g of vinegar. 1 p (atm) 0 4 8 12 V (L) 6 0 12 2 4 8 10 14 16 16 31
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