States of Matter I: Gases

Chemistry, HSSC-KPK First Year. This chapter is split into 37 topics and has 867 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. Kinetic molecular theory & gas pressure 25 questions
2. Pressure and its units 25 questions
3. Boyle's law, Charles' law & absolute zero 25 questions
4. Avogadro's law & molar volume 25 questions
5. Ideal gas equation & the gas constant R 25 questions
6. Mass, density & concentration from the ideal gas equation 25 questions
7. Deviation from ideal behaviour & Van der Waals equation 25 questions
8. Dalton's law of partial pressures 25 questions
9. Liquefaction of gases & critical phenomena 25 questions
10. Plasma: the fourth state of matter 25 questions
11. Pressure unit conversions (atm, torr, mm Hg, Pa, kPa) 25 questions
12. Celsius, Kelvin and Fahrenheit temperature conversions 24 questions
13. Boyle's law numerical problems 23 questions
14. Charles' law numerical problems 22 questions
15. Combined gas equation: numerical problems 23 questions
16. Moles, mass, volume and molecules of gases at STP 27 questions
17. Ideal gas equation: finding pressure and volume 23 questions
18. Ideal gas equation: moles, temperature and the gas constant 20 questions
19. Mass of a gas from the ideal gas equation: numerical problems 20 questions
20. Molar mass and density of gases: numerical problems 25 questions
21. Concentration of a gas in mol per dm3 from c = P/RT 20 questions
22. Dalton's law: partial pressures from composition 22 questions
23. Dalton's law: moles and total pressure in a closed vessel 20 questions
24. Diffusion and effusion of gases: meaning and examples 22 questions
25. Graham's law of diffusion: statement, forms and derivation 20 questions
26. Graham's law numerical problems: relative rates and times of effusion 30 questions
27. Graham's law: determining molar mass and identifying gases 23 questions
28. Experimental verification of Graham's law with NH3 and HCl 25 questions
29. Kinetic gas equation and the root mean square velocity 24 questions
30. Root mean square velocity: numerical problems 27 questions
31. Average kinetic energy of gas molecules and temperature 21 questions
32. Compressibility factor Z: calculation and interpretation 22 questions
33. Van der Waals equation: numerical use of a and b 24 questions
34. Critical temperature and pressure: Table 4.3 and liquefaction conditions 21 questions
35. Linde's apparatus, Joule-Thomson cooling and uses of liquefied gases 24 questions
36. Plasma: states of matter, occurrence and spray and welding applications 20 questions
37. Reading the tables and graphs of the gas laws 20 questions

Sample questions: States of Matter I: Gases

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. Which postulate of the kinetic molecular theory is the reason why a real gas deviates from ideal behaviour at high pressure?

• A. Gas molecules exert no pressure
• B. Collisions between gas molecules are elastic
• C. The volume of the gas molecules is negligibly small compared with the total volume of the gas(correct answer)
• D. Gas molecules are in constant random motion

Answer: C.The assumption that the volume of the molecules is negligibly small compared to the total volume is valid only at low pressures and high temperatures. At high pressure the molecules come closer and their own volume becomes significant, so the gas becomes less compressible than predicted.

Q2. The second postulate giving rise to deviations from ideal behaviour is that:

• A. Collisions are inelastic
• B. The forces of attraction between gas molecules are negligible(correct answer)
• C. Gas molecules have large volumes
• D. The molecules are stationary

Answer: B.The assumption that the forces of attraction between gas molecules are negligible fails at high pressure and low temperature, where the molecules come closer and the attractions become effective, causing the gas to occupy less volume than Boyle's law predicts.

Q3. The condensation of every gas on cooling shows that:

• A. Gases cannot be liquefied
• B. Gas molecules have no mass
• C. Gas molecules are chemically bonded
• D. Attractive forces always exist among gas molecules(correct answer)

Answer: D.Molecules of real gases do exert forces on each other, and the condensation of every gas on cooling shows that attractive forces are always present. These forces are unimportant when the molecules are far apart but become noticeable at high pressure and low temperature.

Q4. In the pressure correction of the Van der Waals equation, the observed pressure P of a real gas is less than the ideal pressure because:

• A. The molecules repel each other strongly
• B. The container expands
• C. Molecules approaching the wall experience a net inward pull from the other molecules, so they strike the wall with less force(correct answer)
• D. The temperature increases near the wall

Answer: C.A molecule in the interior has balanced attractions, but as a molecule approaches the wall it experiences a net inward pull from the unbalanced molecular attraction. This prevents it from contributing its full share to the pressure, so P is less than the ideal pressure.

Q5. Which of the following statements about the two Van der Waals constants is correct?

• A. The constant a accounts for molecular volume and b for attraction
• B. The constant a accounts for intermolecular attraction and b for the finite volume of the molecules(correct answer)
• C. Both constants account for attraction
• D. Both constants account for molecular volume

Answer: B.The Van der Waals equation adds two terms: a/V^2 corrects for the attractive forces between the molecules (constant a) and b corrects for the finite volume of the gas molecules. The two corrections address exactly the two faulty assumptions of the kinetic theory.

Q6. Which of the following is NOT a reason for the deviation of real gases from ideal behaviour?

• A. The gas molecules are in constant random motion(correct answer)
• B. Attractive forces exist between the gas molecules
• C. The attractive forces become effective at low temperature
• D. The volume of the gas molecules is not negligible at high pressure

Answer: A.Constant random motion of the molecules is a valid assumption and is not a cause of deviation. The deviations arise from the finite volume of the molecules at high pressure and from the intermolecular attractive forces that become effective at high pressure and low temperature.

Q7. The partial pressure of a gas in a mixture equals:

• A. The product of its mole fraction and the total pressure(correct answer)
• B. The mole fraction divided by the total pressure
• C. The total pressure multiplied by the molar mass
• D. The total pressure divided by the number of gases

Answer: A.Since the partial pressure of a component is proportional to its share of the total number of moles, it equals the product of its mole fraction and the total pressure. This is why percentage composition data can be used directly with the total pressure.

Q8. According to the kinetic-molecular theory, the pressure exerted by a mixture of different gases should be the sum of the pressures each would exert alone because:

• A. The molecules have identical masses
• B. The gases react with each other
• C. The gas molecules behave independently, so it makes no difference whether the molecules are the same or different(correct answer)
• D. The molecules attract each other strongly

Answer: C.If gases behave according to the kinetic-molecular theory there should be no difference in the pressure-volume-temperature relationships whether the gas molecules are all the same or different, since the molecules behave independently. Each gas therefore exerts the same pressure as if it alone occupied the container.

Q9. Liquid chlorine is used for all of the following EXCEPT:

• A. Killing pathogens in drinking water
• B. As a rocket engine fuel(correct answer)
• C. Bleaching of pulp and textiles
• D. Purification of swimming pools

Answer: B.Liquid chlorine is used for killing pathogens in drinking water, purifying swimming pools, sanitation of industrial waste and sewage, bleaching of pulp and textiles, and in preparing chemicals such as carbon tetrachloride. Rocket engines use liquid oxygen and liquid hydrogen.

Q10. Which of the following is NOT listed as an application of plasma?

• A. Coating alumina onto stainless steel to reduce wear
• B. Use in computers and other electronic equipment
• C. Use in semiconductors, lasers and fluorescent lamps
• D. Liquefying natural gas for transport(correct answer)

Answer: D.Plasma is used in semiconductors, sterilization of medical products, printers, fluorescent lamps, lasers, diamond-coated films, computers and TVs, micro-plasma welding and plasma spraying. Liquefaction of natural gas for transport is an application of the liquefaction of gases, not of plasma.