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**GASES Question 1: 1994 B Free Response**

Sherrie Park AP Chemistry Gangluff, Per. 3/4

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A student collected a sample of hydrogen gas by the displacement of water as shown by the diagram above. The relevant data are given in the following table.

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**A). Calculate the number of moles of hydrogen gas collected.**

To complete this question, you must use the equation PV= nRT correctly. You are already given the following from the table: Given these values, you can now calculate the moles of hydrogen by substituting the numbers into the equation and solving for (n). PV=nRT (721.3 mmHg)(0.09 L) = (x)(62.4 L · Torr · K-1 · mol-1) (298 K) = 3.49 x 10^-3 mol H2 P (Pressure)= 745 mm Hg – 23.8 mm Hg = 721.2mm Hg ** The equilibrium vapor pressure must be subtracted from atmospheric pressure to find the pressure of hydrogen gas collected. V (volume)= 90.0 mL x (1 L/ 1000mL) = 0.09 L ** must be in Liters Gas constant (R) = 62.4 L · Torr · K-1 · mol-1 **This constant is specifically used because the pressure is calculated in Torrs. Temperature (T) = 25 C = 298 K **Must be converted into Kelvins (absolute temperature)

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**B). Calculate the number of molecules of water vapor in the sample of gas.**

In order to answer this question, start by using PV=nRT to find the moles of water vapor. Then convert moles into molecules (or atoms) by multiplying the number of moles received with Avogradro’s number: x 10^23. P = 23.8mmHg V = 90.0ml/1000ml = 0.09 L N = x mol of H2O R = 62.4 L · Torr · K-1 · mol-1 T = 25 C = 298 K (1.15x10^-4 mol H2O)(6.022 x 10^23) = 6.93 x 10^19 molecules of H2O PV = nRT (23.8mmHg)(0.09 L) = (x)(62.4 L · Torr · K-1 · mol-1)(298 K) =1.15x10^-4 moL H2O

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C). Calculate the ratio of the average speed of the hydrogen molecules to the average speed of the water vapor molecules in the sample. Equation for Average Kinetic Energy: KE = (1/2)mv^2 The average kinetic energies of H2 and H2O will be equal to one another. Therefore… (MassH2)(VelocityH2)^2 = (MassH2O)(VelocityH2O)^2 The masses are the molar masses of each molecule: 2 g H2 and 18 g H2O 2(VH2)^2 = 18(VH2O)^2 VH2/VH2O = Square root (18/2) = 3

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**D). Which of the two gases, H2 or H20, deviates more from ideal behavior? Explain your answer.**

In order to answer this question, you need to know the deviations from ideal gas law behavior. Deviations are greater if: 1). Intermolecular forces of gas molecules are greater. 2). Mass (and subsequently volume) of gas molecules is greater. Basically, the larger the molecule is, the more it deviates from ideal behavior. In this case, H2O deviates more from ideal behavior. The is because the volume of the H2O molecule is greater than the volume of the H2 molecule. It is also the larger molecule. Additionally, the intermolecular forces among the H2O molecules are stronger than those among the H2 molecules. Since it has a great number of electrons, it has a greater Van Der Waals Attraction.

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Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Gases.

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