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Gas Stoichiometry

Gas Stoichiometry. Unit 11. Objectives. 11.1 State and use Dalton’s Law of Partial Pressures 11.2 State Avogadro’s Principle 11.3 Use the ideal gas law to solve problems and know the variables of the ideal gas law. 11.1 Dalton’s Law of Partial Pressures.

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Gas Stoichiometry

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  1. Gas Stoichiometry Unit 11

  2. Objectives 11.1 State and use Dalton’s Law of Partial Pressures 11.2 State Avogadro’s Principle 11.3 Use the ideal gas law to solve problems and know the variables of the ideal gas law

  3. 11.1 Dalton’s Law of Partial Pressures • When gases were discussed in Unit 10, it was mentioned that pressure was measured with the collisions gas particles underwent. • The total pressure is a sum of all of those collisions. • Therefore, Dalton’s Law states that the pressure of each gas can be added to determine the total pressure.

  4. Dalton’s Law of Partial Pressures • Though Dalton’s Law seems fairly basic, it is extremely useful when collecting a gas. • When most experiments are performed, the gases produced are allowed to escape. • However, if it is the gas that needs to be analyzed, the gas most be collected. • The collection of this gas is typically done over water.

  5. Collecting a Gas over Water • Collecting a gas over water requires a sealed container with a tube into a tank of water. • In the tank of water, an inverted tube is filled with water. • As the reaction progresses, the gas produced follows the tube into the water chamber and up the inverted tube.

  6. Dalton’s Law • Dalton’s Law comes into play because a small amount of water with change to a gas in the container. • Therefore, the gas collected and water vapor combine to give the pressure. • That pressure is equal to the atmospheric pressure outside of the tube. • Therefore, the following equation applies: Patmosphere = Pgas + Pwater

  7. 11.2 Avogadro’s Principle • Up to this point, we have examined gases under the assumption that we always held the same number of moles in the container. • This is not always the case. • Just as a relationship was determined between pressure, volume, and temperature, a relationship was determined between the number of moles and volume.

  8. Avogadro’s Principle • According to Avogradro’s Principle, if the number of moles increase, the volume also must increase assuming constant temperature and pressure. n = moles

  9. Avogadro’s Principle • Using this principle, it was determined that at STP (standard temperature and pressure), one mole of a gas would always take up the same volume. At 0°C and 1 atm, 1 mole will take up 22.4 liters.

  10. 11.3 Ideal Gas Law • With the inclusion of the mole into the relationships of gases, it could be added to the combined gas law as well. K represents a constant

  11. Ideal Gas Law • Upon further analysis, it was determined that the constant could be calculated and was the same for each container. • Assume STP conditions: • 1 Mole • 22.4 Liters • 273.15 K • 1 atm k = 0.0821

  12. Ideal Gas Law • The constant was changed to R and requires specific units to be used. • There are two commonly used values for R: 0.0821 or 8.314 Required Units: Volume: Liters Amount: moles Temperature: Kelvin Pressure: atm or kPa

  13. Ideal Gas Law The equation for the Ideal Gas Law is: PV=nRT The value of R is chosen based on the units on the pressure.

  14. Gas Stoichiometry • With Dalton’s Law and the Ideal Gas Law, it is now possible to incorporate gases into stoichiometry. • Recall the steps to a stoichiometry problem: • Balance the equation • Convert to moles • Use the mole ratio • Convert to the desired units

  15. Gas Stoichiometry • The ideal gas law allows us to calculate for moles and liters. • Moles are needed for step 2 of stoichiometry. • Liters could be found in step 4 of stoichiometry. • Solving a gas stoichiometry problem requires knowing if the ideal gas law is used first or last. • If given liters, use the ideal lawfirst. • If asked for liters, use the ideal lawlast.

  16. Gas Stoichiometry • Consider the following: Zinc reacts with hydrogen chloride to produce zinc chloride and hydrogen gas at 25°C and 0.98 atm. If 15 liters of hydrogen gas were produced, how many grams of zinc were used?

  17. Zinc reacts with hydrogen chloride to produce zinc chloride and hydrogen gas at 25°C and 0.98 atm. If 15 liters of hydrogen gas were produced, how many grams of zinc were used? • Since the problem asks for liters, the ideal gas law must be used first so moles can be found. • Look at the numbers and determine what is known: • Pressure: 0.98 atm • Volume: 15 liters • Amount: ? Moles • R: 0.0821 • Temp.: 25°C  298 K

  18. Zinc reacts with hydrogen chloride to produce zinc chloride and hydrogen gas at 25°C and 0.98 atm. If 15 liters of hydrogen gas were produced, how many grams of zinc were used? • Solve for moles to complete step two of stoichiometry. PV=nRT 0.98 atm x 15 L = X moles x 298 K x 0.0821 X = 0.60 moles of hydrogen gas • Since the balanced equation is as follows: Zn + 2HCl  ZnCl2 + H2 We can use our regular steps to finish the problem.

  19. Gas Stoichiometry • Consider the following: Zinc reacts with hydrogen chloride to produce zinc chloride and hydrogen gas at 25°C and 0.98 atm. If 15 grams of zinc were used, how many liters of hydrogen gas could be collected over water?

  20. Zinc reacts with hydrogen chloride to produce zinc chloride and hydrogen gas at 25°C and 0.98 atm. If 15 grams of zinc were used, how many liters of hydrogen gas could be collected over water? • This problem is different because it starts with grams. • Therefore, the ideal gas law will be used last. • Also, notice that it says “collected over water.” • This means that Dalton’s Law must be used before the ideal gas law.

  21. Zinc reacts with hydrogen chloride to produce zinc chloride and hydrogen gas at 25°C and 0.98 atm. If 15 grams of zinc were used, how many liters hydrogen gas could be collected over water? • First, perform the stoichiometry to determine moles of hydrogen gas. Zn + 2HCl  ZnCl2 + H2

  22. Zinc reacts with hydrogen chloride to produce zinc chloride and hydrogen gas at 25°C and 0.98 atm. If 15 grams of zinc were used, how many liters hydrogen gas could be collected over water? • Now that we know the moles of hydrogen gas, we can use the ideal gas law to find the liters. However, we first must use Dalton’s Law to find the actual pressure. Patmosphere = Pgas + Pwater 0.98 atm = Pgas + 0.031 atm Pgas = 0.949 atm The pressure of water is listed on the back of the Periodic Table.

  23. Now we can use the ideal gas law. P = 0.949 atm V = X liters n = 0.23 moles R = 0.0821 T = 25°C  298.15 K PV=nRT 0.949 atm x X L = 0.23 moles x 0.0821 x 298.15 K V = 5.9 liters

  24. This concludes the tutorial on measurements. • To try some practice problems, click here. • To return to the objective page, click here. • To exit the tutorial, hit escape.

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