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Gas Laws

Gas Laws. Review: Ideal Gas Law. Definition : Describes the interrelationships between the four variables that characterize a gas sample at a given time: pressure (P), volume (V), absolute temperature (T) and quantity of gas (n), and the gas constant (R). Formula : PV = nRT

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Gas Laws

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  1. Gas Laws

  2. Review: Ideal Gas Law • Definition: Describes the interrelationships between the four variables that characterize a gas sample at a given time: pressure (P), volume (V), absolute temperature (T) and quantity of gas (n), and the gas constant (R). • Formula: PV = nRT • Units: P = kPa V = litres (L) n = moles (mol) R = 8.31 T = kelvin (K)

  3. Review: Molar Mass & the Ideal Gas Law • Molar mass of a gas tells us information about the density of a gas providing the temperature and pressure conditions remain the same • The greater the molar mass of a gas sample the greater the gas's density Ex: CO2(s) has a greater molar mass and therefore greater density than air so it “sinks”

  4. If: PV = nRt • Then: PV = mRT M Or: M = mRT PV

  5. Ideal Gas Law Questions 1) Calculate the number of moles of methane (CH4) gas contained in a sample with a volume of 500 mL at 35 °C and 210 kPa 2) What is the molar mass of a gas if 2.6 L of this gas has a mass of 5.4 g at 100 °C and 26.6 kPa? 3) What is the volume of a weather balloon if it contains 10 mol of air at 75.5 kPa and -45 °C

  6. Review: General Gas Law • Definition: Describes the relationship between the four variables that describe gases: “P”, “V”, “T” and “n”. It can be used to predict the final conditions of a gas once the initial conditions have been modified • Formula: P1V1 = P2V2 n1 T1 n2 T2

  7. Why the General Gas Law is Great • Because the general gas law is a combination of all the individual gas laws it can be used to deduce each of the gas laws that make it up • Example: Moles (n) and Pressure (P) are kept constant in a particular situation P1V1 = P2V2 n1 T1 n2 T2 -P & n cancel each other out - Law Remaining = Charles’ Law

  8. Stoichiometry of Gases • Definition: Calculation method based on the ratios between the quantities of gas involved in a chemical reaction. Used to predict the quantity of a reactant or product if at least one of the components is a gas.

  9. History • Gay-Lussac’s discovery of the combined gas law became a fundamental law of chemistry: • When gases react, the volumes of the reactants and the products, measured at constant temperatures and pressures, are always present in the form of simple whole number ratios

  10. Example Propane (C3H8) burns in the air according to the following equation: C3H8(g) + 5 O2 (g)→ 3 CO2(g) + 4 H2O(g) a) What volume of oxygen (O2) is needed for the combustion of 35.0 L of propane if the volumes are measured at the same conditions of temperature and pressure?

  11. b) What volume of CO2 will be produced if 155 g of propane reacts with enough oxygen at STP?

  12. Dalton’s Law • Definition: Also known as “Law of Partial Pressures”, states that at a given temperature, the total pressure of a mixture of gases equals the sum of the partial pressures of each of the gases • Connection to Kinetic Theory of Gases: Hypothesis 3: Gases do not exert pressure on each other so their pressures are the same alone or in combination

  13. Formula: Pt = P1 + P2 + Pn • Pressures can be in mmHg or kPa • A manometer (device used to measure pressures close to atmospheric) cannot measure partial pressures of combination gases, only the total pressure • To calculate partial pressures individually the molar relationships must be used

  14. Partial Pressure of a Gas: P1 = n1 x Pt nt Where P1 = Pressure of Gas 1 (mmHg or kPa) n1 = Moles of Gas 1 (mol) nt = Moles of the Total Quantity of Gas (mol Pt = Total pressure of the mixture (mmHg or kPa)

  15. Example At a given temperature, a mixture of gas contains 3.35 mol of neon (Ne), 0.64 mol of argon (Ar) and 2.19 mol of xenon (Xe). What is the partial pressure of xenon if the total pressure of the mixture is 200.0 kPa?

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