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Self Test on States of matter, intermolecular forces, and gas laws

Short essay questions

1.

What is the origin of the earth's atmospheric pressure? What does the term"1 atmosphere" mean? The mass of air (gas) that extends from the surface of the earth to the edge of space has a weight. This weight exerts a force at sea level. The weight of air over a square inch at sea level is 14.7 pounds, 14.7 lbs/per square inch, (14.7 psi)

2.	Describe dipole-dipole forces and London forces. Describe how each arises. Tell which one is stronger. Give examples of molecules for both types of forces. molecule. Visit the webpage on intermolecular forces

3.

Tell which type of interaction ( dipole-dipole forces or London forces)exists between molecules in the following table. molecule type of intermolecular force molecule type of intermolecular force N2 London force----------------------------- CH3Cl Dipole-dipole------------------------------------------- CO2 London force ----------------------------------- SO2 Dipole-dipole -----------------------------------------------

4.

What is the reason that liquids have different normal boiling points? Intermolecular forces are different for different molecules. When the intermolecular forces are weak the boiling points are low. When the intermolecular forces are strong the boiling points are high. What is a "normal" boiling point? The normal boiling point is the temperature at which the vapor pressure equals one atmosphere. Do liquids only boil at the "normal" boiling point? A liquid can boil at any temperature. The requirement is that the vapor pressure has to equal the opposing pressure of the atmosphere. When the atmospheric pressure goes up so does the boiling point. When the atmospheric pressure drops the boiling point goes down. Boiling water is easier on a mountain top. The problem is that the water isn't as hot as it would be at sea level. Pressure cookers are used because the boiling point for water is higher at the increased pressure in the pressure cooker. Food is freeze dried by placing it in a low pressure environment so the water "boils" off without heating to an elevated temperature.

5.

What is meant by the term "vapor pressure"? All liquids have some number of molecules with enough energy to escape into the gas state over the surface of the liquid. These gas state molecules exert a pressure. The pressure these molecules exert is called the vapor pressure. The vapor pressure is the gas partial pressure that gas state molecules exert Describe how intermolecular forces and temperature influence the vapor pressure of a substance.   The temperature controls the kinetic energy of molecules in the liquid. When the temperature is low the average kinetic energy is low. This results in few molecules with enough em=energy to escape into the gas state. Low temperatures match low vapor pressure. The intermolecular forces influence the vapor pressure. When the forces are strong molecules have a difficult time escaping. Strong interactions give low vapor pressures. Weak intermolecular forces are the opposite. Weak forces result is high vapor pressures.

6.

Do solids and liquids both show a vapor pressure? Explain Yes, both solids and liquids show a vapor pressure. Some molecules will have enough energy to escape from the solid into the gas. The vapor pressure for solids is lower than the vapor pressure of a liquid. Dry ice is solid CO2. Dry ice goes directly from the solid into the gas. This is called sublimation.

7.

Vapor pressure values for, CH3Br, methyl bromide are given in the table below. How do the intermolecular forces between methyl bromide, CH3Br, molecules compare with the strength of intermolecular forces between water, H2O molecules. Use the data for vapor pressures to support your answer. CH3Br, Temperature in oC vapor pressure in mm Hg -63oC 100 -38oC 400 -24oC 760 The vapor pressure for methyl bromide is 760 mm Hg at a very low temperature. The normal boiling point is -24oC. This is much lower than the boiling point for water. The intermolecular forces in CH3Br must be very weak compared to the intermolecular forces in water.

8.

Why does the ideal gas equation give the same volume for a mole of any gas? PV = nRT

Does the Ideal gas law pay attention to the fact that molecules of methane CH4 and butane CH3CH2CH2CH3 have different volumes? The ideal gas equation ignores actual molecule sizes. The volume used in calculations is the volume of the container. No corrections are made for the "real" volume of gas molecules. This is no problem if the pressure is low and relatively few molecules are in the container. The neglect of the actual molecule volume becomes important when the gas pressure is high. that means more than 5 or ten atmospheres.

9.

Which molecule in the following pairs has stronger intermolecular forces? Identify the types of forces present for each molecule, London forces, LF; dipole-dipole forces,DD; hydrogen bonding, HB. molecule pair types of intermolecular force molecule pair types of intermolecular force H2 or N2 LF in both H2 and N2 The greater number of electrons in N2 mean it has stronger London forces. CH3Cl or CH4 LF in  CH4 LF and DD forces in CH3Cl There are stronger forces between CH3Cl molecules SO2 or CO2 LF and DD forces in SO2 LF forces in CO2 There are stronger forces between SO2 molecules H2O or H2S LF, HB, and DD forces in H2O LF and DD forces in H2S There are stronger forces between H2O molecules

Multiple choice

1.	Which of the following pairs of shows dipole-dipole forces.
	a. NaCl and HCl b. HF and H2 c. H2 and H2 d.HF and HF

2.	Which of the following exists as a covalent network solid.
	a. Cu
	b. NaCl
	c. CH4
	d.diamond, C

3.	How many mols of CO2 gas are in the following gas sample? V = 2.24 L ; P = 1.0 atm ; T = 310 K Recall PV = nRT; R = 0.0821 L atm/ mol K
	a. 0.0094 mol
	b. 0.094 mol
	c. 0.94 mol
	d. 0.000094 mol

4.	Which of the following displays hydrogen bonding.
	a. CH4
	b. CH3OH
	c. CH3OCH3
	d. CH3CH2CH2CH3

5.	What is the molar volume of an ideal gas at STP?
	a. it depends on the gas
	b. it is always 22.4 L
	c. it depends on the volume of the container
	d. cannot say the molecular formula is needed

6.	Which of the following substances has the lowest melting point?
	a. gold, Au
	b. sand, SiO2
	c. butane, CH3CH2CH2CH3 , C4H10
	d.water, H2O

7.	Which of the following gases has the lowest density?
	a. methane, CH4
	b. ethane, CH3CH3
	c. propane, CH3CH2CH3
	d. butane, CH3CH2CH2CH3

8.	Which of the following substances has the strongest London forces?
	a. methane, CH4
	b. ethane, CH3CH3
	c. propane, CH3CH2CH3
	d. butane, CH3CH2CH2CH3

9.	What is the final pressure for a gas that was initially at 0.95 atm at 25oC if the temperature is increased to 125oC?
	a. (0.95 atm)(298 K/398 K), it goes up
	b. (0.95 atm)(125oC/25oC), it goes up
	c. (0.95 atm)(25oC/125oC), it goes up
	d. (0.95 atm)(398 K/298 K), it goes up

10.	What is the final volume for a sample of oxygen, O2, that was initially at 0.65 atm at 25oC with a volume of 600 mL, if the pressure is increased to 0.85 atm at 25oC?
	a. (600 mL)(0.65 atm/0.85 atm); the volume goes down
	b. (600 mL)(0.85 atm/0.65 atm); the volume goes up
	c. (0.65 atm/0.85 atm) /(600 mL); the volume goes up
	d. the volume stays the same

11.	What is the pressure in atmospheres when the pressure is 440 mm Hg?
	a. (440)(760)
	b. (440)/(760)
	c. (760)/(440)
	d. 760 - 440

12.	What is the volume of 34 grams of H2 gas at STP?
	a. 22.4 L
	b. (22.4 L)x(34)
	c. (22.4 L)x(34)(1/2)
	d. N and O

13.	Which of the following is arranged in order of increasing boiling points.
	a. Kr, Ne, He
	b. Br2, F2, Cl2
	c. H2Se, H2S, H2O
	d. H2O, He, N2

14.	Which of the following will have stronger London forces.
	a. H2
	b. He
	c. Ne
	d. Kr

15.	Which of the following molecules shows the greatest effects of hydrogen bonding?
	a. H2S
	b. CH4
	c. NH3
	d. H2O

  Online Introductory Chemistry