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# 13.3 Up Up And Away

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### 13.3 Up Up And Away

1. 1. Matter Very Simple Up, Up and Away
2. 2. Gas Behaviour <ul><li>So far, we have found out: </li></ul><ul><ul><li>Volume is proportional to absolute temperature – Charles’ Law </li></ul></ul><ul><ul><li>P ressure is proportional to absolute temperature – Pressure Law </li></ul></ul><ul><ul><li>For a fixed amount of gas, the product of Pressure x Volume </li></ul></ul><ul><ul><li>is constant – Boyle’s Law </li></ul></ul><ul><ul><li>This may be written as P  1/V </li></ul></ul>
3. 3. The Gas Laws <ul><li>We will find out: </li></ul><ul><ul><li>Why gases provide buoyancy </li></ul></ul><ul><ul><li>How all of the gas laws can be combined to give the ideal gas relationship </li></ul></ul>
4. 4. Buoyancy <ul><li>Things are said to be buoyant when they float… </li></ul><ul><li>This may be a boat in water </li></ul><ul><li>The force keeping it up is called ‘upthrust’ </li></ul><ul><li>It is due to the weight of the displaced medium (in this case water) and is equal to weight for a floating object </li></ul>
5. 5. A load of hot air! <ul><li>Gases can rise when in the presence of other gases. </li></ul><ul><li>This is due to the displacement of a denser gas </li></ul>Helium Weight of Helium Upthrust = Weight of displaced air The resultant force is the weight of the displaced gas minus the weight of the actual gas.
6. 6. One Law for all Gases But what happens if we combine all 4 laws..? Introducing the ‘Amount Law’…
7. 8. Example 1 <ul><li>A 5 mol sample of nitrogen exerts a pressure of 150 000 Pa at a temperature of 373 K. What is the volume of this sample? </li></ul><ul><li>Hint… pV = nRT </li></ul><ul><li>Where p = Pressure in Pascal or Nm -2 </li></ul><ul><li>V = Volume </li></ul><ul><li>n = number of moles </li></ul><ul><li>R = molar gas constant 8.31 J mol -1 K -1 </li></ul><ul><li>T = Temperature in Kelvin </li></ul><ul><li>Introducing Avogadro's Constant = Number of gas particles in 1 mole </li></ul>
8. 10. Example 2 <ul><li>The temperature is changed to 273 K and the pressure drops to 100 000 Pa. What is the volume now? </li></ul><ul><li>Hint… pV = nRT </li></ul>
9. 12. The Gas Laws <ul><li>We will find out: </li></ul><ul><ul><li>Why gases provide buoyancy </li></ul></ul><ul><ul><li>Resultant Force </li></ul></ul><ul><ul><li>= Weight of displaced fluid – Weight of Displacing Gas </li></ul></ul><ul><ul><li>How all of the gas laws can be combined to give the ideal gas relationship </li></ul></ul><ul><ul><li>pV = nRT </li></ul></ul>