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Chapter 5

Chapter 5. Measurements on Gases Chemistry: Principles and Reactions Jay Adams Fall 2007. Four things we look at. Pressure: the amount of force a gas exerts or is subject to Volume: The amount of space a gas takes up Temperature: The speed at which molecules of a gas move

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Chapter 5

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  1. Chapter 5 Measurements on Gases Chemistry: Principles and Reactions Jay Adams Fall 2007

  2. Four things we look at • Pressure: the amount of force a gas exerts or is subject to • Volume: The amount of space a gas takes up • Temperature: The speed at which molecules of a gas move • Number of moles (or atoms) of a gas: this is the weirdest property of gases

  3. Properties of Gases • They are easily compressed—ever buy a can of propane for the grill? • They expand to take the shape of their containers and fill all available space—this is why you can smell bacon in your bedroom when your mom cooks it in the kitchen • They take up more space than liquids and solids

  4. Measuring Gases • The volume of a gas is typically the volume of its container (see property #2). Usually we express this in L or cc (1 L = 1000 cc) • The amount of a gas is typically expressed as either the mass (in g) or the number of moles (n) • The temperature of a gas must be converted to K before any calculation can be done. • (Tk = Tc + 273)

  5. Pressure • Definition: force per unit area (usually pounds per square inch—read your tire) • What does 44psi mean? • Other expressions: atm (the pressure exerted by a gas at 273 K) • Other expressions: mmHg (760 mmHg = 1 atm) • Look at example 5.1 (page 105)

  6. The Ideal Gas Law • Liquids vary greatly, as do solids. But most gases are pretty similar, physically. • Volume is directly proportional to amount of gas. • What does directly proportional mean? • What is a constant? • Write this equation.

  7. The ideal gas law (2) • Volume is directly proportional to temperature (in K) • Write this equation. • Volume is inversely proportional to pressure • What does inversely proportional mean? • Write this equation.

  8. Three equations: • You should have these: • V = kn • V = k T • V = k/P • If you put them all together, you get: V = knT /P

  9. Avogadro’s Law • Avogadro figured out this neat property of gases: • All gases at the same temperature and pressure contain the same number of moles, no matter what they are. • This means that the constant from the previous equation must be the same for all gases. • He also fixed STP at 0°C and 1 atm

  10. The ideal gas law (3) • The ideal gas law is given as: • PV = nRT • P,V, n, and T are variable. • R is always, every single time, the same thing • 0.0821 L*atm / mol*K • If you need to know where that number came from, read page 107

  11. PV = n(0.0821 L*atm / mol*K) T • You can see from the units that, for the equation to work, you must use the following units: • Volume in L • Pressure in atm • N in moles • T in Kelvin • This would be a good equation to memorize now.

  12. Three kinds of problems you use it for • Finding the final state of a gas if you know the conditions and its initial state • Finding one of the four variables • Finding the molar mass or density of a gas

  13. A Problem • If I start with a gas at 25°C and 1.00 atm, what would the pressure be if I heat the sample to 95°C? • What stays constant?

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