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CHEMISTRY. IT’S A GAS!! Chapter 13. Units of Pressure. 1atm = 101.3 kPa = 760 torr = 760 mm Hg = 14.7 psi Measured with a barometer or manometer to measure pressure less than atmospheric. Assumptions About Properties of Gases – Kinetic Theory p.475-477.

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Chapter 13

units of pressure
Units of Pressure
  • 1atm = 101.3 kPa = 760 torr = 760 mm Hg = 14.7 psi
  • Measured with a barometer
  • or manometer to measure pressure less than atmospheric
assumptions about properties of gases kinetic theory p 475 477
Assumptions About Properties of Gases – Kinetic Theory p.475-477
  • Small particles with lots of space between-insignificant volume
  • Compressible
  • High kinetic energy – random motion and straight paths
  • Elastic collisions
  • No attractive or repulsive forces between particles You know what they say about assuming!! More on that later. 
variables that describe gases
Variables That Describe Gases
  • Pressure (P) – kPa, atm, Torr, mm Hg, psi, bars or mbars
  • Volume (V)– liters
  • Temperature (T)– Kelvin scale
  • Moles (n) – number of moles
gas laws
Gas Laws
  • Allow us to predict behavior of gases under specific conditions.
  • Help us understand everyday applications of gases
  • Why do we have to check air pressure in tires?
  • Why did my balloon shrink in the cold?
  • Why does warm soda fizz so much?
factors affecting gas pressure
Factors Affecting Gas Pressure
  • Amount of gas – number of particles/moles
  • Volume – space gas takes up
  • Temperature – increases or decreases kinetic energy of particles resulting in more or fewer collisions
boyle s law p 447
Boyle’s Law p.447
  • Describes pressure-volume relationship of gases at constant temperature
  • States that for a given mass of gas, as pressure increases, volume decreases and vice versa
  • What kind of relationship is this?
  • How can it be expressed mathematically?
  • P1V1 = P2V2
charles law p 451
Charles’ Law p.451
  • Describes the relationship between volume and temperature at constant pressure.
  • States that for a fixed mass of gas, as temperature increases, volume increases
  • What kind of relationship is this?
  • How can this be expressed mathematically?
  • V1/T1 = V2/T2
gay lussac s law
Gay-Lussac’s Law
  • Describes relationship between temperature and pressure with constant volume
  • States that the pressure of a gas will increase with increased temperature and vice versa
  • What kind of relationship is this?
  • How can it be expressed mathematically?
  • P1/T1 = P2/T2
combined gas law p 464
Combined Gas Law p. 464
  • Combines Boyle’s, Charles’ and Gay-Lussac
  • Allows for calculations where none of the variables is constant
  • P1V1/T1 = P2V2/T2
avogadro s hypothesis law p 455
Avogadro’s Hypothesis (Law) p.455
  • Equal volumes of gases at the same temperature and pressure contain equal numbers of particles
  • Gases at constant pressure and temperature have volume that is directly proportional to number of moles.
  • V1/n1 = V2/n2
  • One mole (6.02 x 1023 particles) of any gas at STP occupies a volume of 22.4 L
ideal gas law p 458
Ideal Gas Law p.458
  • Incorporates the fourth variable of gases – number of moles
  • When “n” is included in combined gas law a constant is recognized and symbolized as “R” with value of 0.08206 Latm/Kmol
  • “R” is known as the ideal gas constant
  • New mathematical expression PV=nRT
ideal gas p 458
Ideal Gas p.458
  • One that follows all gas laws at all conditions of pressure and temperature
  • No such gas – however, real gases behave as ideal gases under most conditions of pressure and temperature
  • Real gases can be liquefied and sometimes solidified – ideal gases cannot
real vs ideal p 478
Real vs Ideal p.478
  • Kinetic theory assumptions – no attraction among particles and no volume
  • Assumptions are incorrect!
  • If true, would not be able to liquefy or solidify gases
  • Gases definitely have volume
dalton s law of partial pressures p 464
Dalton’s Law of Partial Pressuresp. 464
  • States at constant volume and temperature the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures
  • Expressed mathematically

P1 + P2 + P3…= Ptotal

graham s law
Graham’s Law
  • Describes diffusion and effusion of gas molecules
  • States that rate of effusion is inversely proportional to the square root of the gases’ molar masses
  • Also found to be true for diffusion of gases
  • Simply put, means small gas molecules move faster than heavier gas molecules
  • Can be expressed mathematically