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Chapter Objectives

- Describe the physical properties of gases.
- Use the ideal gas law for calculating changes in the conditions of gases.
- Use the concept of partial pressure to work with mixtures of gases.
- Perform stoichiometric calculations for reactions involving gases as reactants or products.

Pressure

- Pressure:

Gas Pressure

- Pressure is the result of molecular collisions between gas molecules and container walls.

- A barometer is used to measure pressure.

Units of Pressure and STP

- 1 torr = 1 mm Hg
- 1 atm = 760 torr= 760 mm Hg (exactly)
- 1 atm = 101,325 Pa (exactly)
- 1 atm = 14.7 psi
- At sea level, atmospheric pressure is?
- Standard Temperature and Pressure Conditions (STP):

Empirical Gas Laws

- Gases behave simply under “moderate” conditions with respect to four things:

Boyle’s Law

PV = constant P1V1 = P2V2

For a given amount of gas at a constant temp.

Charles’s Law

V/T = constant V1 / T1=V2 / T2

For a given amount of gas at a given pressure

WARNING – Temperature must be absolute (Kelvin Units) for ALL gas laws.

Examples

- A volume of oxygen gas occupies 38.7 mL at 751 mm Hg at 21°C. What is the volume if the pressure changes to 359 mm Hg and temp. remains constant?
- You prepared carbon dioxide by adding HCl(aq) to marble chips (CaCO3). According to your calculations, you should obtain 79.4 mL of CO2 at 0°C and 760 mm Hg. How many milliliters of gas would you obtain at 27°C?

Combined Gas Law

PV = constant P1V1 = P2V2

TT1 T2

(for a given molar amount of gas)

Example:

Divers working from a North Sea oil drilling platform experience pressures of 5.0x101atm at a depth of 5.0x102m. If a balloon is inflated to a volume of 5.0L (the volume of a lung) at that depth at a water temperature of 4.0°C, what would the volume of the balloon be on the surface (1.0 atm pressure) at a temperature of 11°C?

Avogadro’s Law

- Relates volume and molar amount.
- One mole of any gas contains how many molecules?
- At STP, Vm =

Ideal Gas Law

- We can combine the empirical gas laws into one equation called the Ideal Gas Law
- There must be a proportionality constant that governs relationships of Empirical Gas Laws = R
- R is the universal gas law constant.
- R = 0.0821 L atm/mol K

The Ideal Gas Law

- The ideal gas law is the quantitative relationship between pressure, volume, moles gas present, and the absolute temperature.
- Units are critical!

Ideal Gas Law Examples and Applications

- A 50.0-L cylinder of nitrogen, N2, has a pressure of 17.1 atm at 23°C. What is the mass of nitrogen in the cylinder?
- What is the density of methane gas (natural gas), CH4, at 125°C and 3.50 atm?

Gas Stoichiometry

Example:

The following reaction can be used to produce pure oxygen gas:

2 H2O2 (l) 2 H2O (l) + O2 (g)

Suppose I used 0.01 moles of H2O2. How many liters of oxygen gas will I get at 298K and 1 atm?

Gas Stoichiometry

Example:

How many liters of chlorine gas, Cl2, can be obtained at 40°C and 787 mmHg from 9.41 grams of HCl according to the following reaction?

2 KMnO4 (s) + 16 HCl (l)

8 H2O (l) + 2 KCl (aq) + 2 MnCl2 (aq) + 5 Cl2 (g)

Dalton’s Law of Partial Pressures

- Partial pressure –
- Dalton’s Law of Partial Pressures –

PT = PA + PB + PC + …

Calculation of Partial Pressure of Gas in a Mixture

A 100.0 mL sample of air exhaled from the lungs is analyzed and found to contain 0.0830 g N2, 0.0194 g O2, 0.00640 g CO2, and 0.0041 g water vapor at 35°C. What is the partial pressure of each component and the total pressure of the sample?

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