This set of Class 12 Chemistry Chapter 7 Multiple Choice Questions & Answers (MCQs) focuses on “P-Block – Group 18 Elements”.
1. Helium is the second most abundant gas in the universe.
Explanation: Helium is the second element in the periodic table and has an atomic number of 2. Helium is formed as the result of the fusion of two hydrogen atoms. Helium is the most abundant gas in the universe (23% as compared to 76% of hydrogen).
2. Which of the following noble gases do not occur in the elemental state in the atmosphere?
Explanation: Radon is the last-named member of the 18th group of elements in the periodic table. It is radioactive in natureand as a result, all other noble gases except radon occur in the elemental state in the atmosphere.
3. Which of the following is the second most abundant noble gas in the atmosphere?
Explanation: Argon is the most abundant noble gas in the atmosphere, accounting for about 0.934% by volume. Neon is the second most abundant noble gas in the atmosphere (about 0.00182%) followed by helium as the third most abundant noble gas (about 0.000524%).
4. The main commercial source of helium is natural gas.
Explanation: The main commercial source of helium is natural gas which mainly contains hydrocarbons along with varying amounts of carbon dioxide, nitrogen, hydrogen sulphide and helium (2-7%).
5. Which of the following scientists first discovered helium in the solar spectrum during a total solar eclipse?
a) Lord Rayleigh
b) J N Lockyer
c) William Ramsay
d) Antoine Lavoisier
Explanation: J N Lockyer first discovered helium in the solar spectrum during a total solar eclipse on 18th August, 1868. Later on, in 1895, it was discovered on the planet Earth by a scientist named William Ramsay.
6. What is the half-life of radon?
a) 10 days
b) 4.56 days
c) 3.82 days
d) 5.46 days
Explanation: Half-life of a radioactive substance refers to the amount of time that it takes for half of the radioactive atoms in a sample to decay into a more stable form. Radon has a half-life of 3.82 days.
7. What are the decay products of radium?
a) Radon and oxygen
b) Radon and nitrogen
c) Lanthanum and oxygen
d) Radon and Helium
Explanation: Radium is a radioactive element with the atomic number 88. It is the 6th element in group 2 of the periodic table. Radium (Ra) on radioactive decay, gives Radon (Rn) and Helium (He) as the products.
8. Which of the following noble gases is not obtained via fractional distillation?
Explanation: Neon, argon, krypton and xenon are obtained by fractional distillation of air. Fractional distillation of liquid air gives oxygen, nitrogen and a mixture of noble gases. The individual noble gases are then separated by adsorption over coconut charcoal which adsorbs different gases at different temperatures.
9. Which of the following statements is incorrect about noble gases?
a) They are monoatomic
b) They are colourless
c) They are odourless
d) They all have an outer electronic configuration of ns2np6
Explanation: The general outer electronic configuration for noble gases is ns2np6. But, for helium, the outer electronic configuration is 1s2 as it its atomic number is only 2. All noble gases are monoatomic, odourless and colourless.
10. What is the ratio of the molar heat at constant pressure to the molar heat at constant volume for a noble gas?
Explanation: All noble gases are monoatomic in nature due to their stable outer electronic configuration. The ratio of their specific or molar heat at constant pressure to the specific or molar heat at constant volume is equal to 1.67, which is the same for all monoatomic gases.
11. What is the electron gain enthalpy for noble gases?
a) > 0
b) < 0
c) = 0
d) It is not defined for noble gases
Explanation: Noble gases have completely filled subshells. As a result, there is no vacant room in their valence shell and hence the additional electron has to be placed in an orbital of the next higher shell. Therefore, energy has to be supplied in order to add an electron and so, the electron gain enthalpy of noble gases is positive (> 0).
12. What is the boiling point of Helium?
a) 7.8 K
b) 0 K
c) 4.2 K
d) 3.7 K
Explanation: The melting and boiling points of noble gases are very low. This is because the atoms are held together by very weak Van der Waals forces of attraction in both liquid and solid states. Helium has the lowest boiling point, equal to 4.2 K, among all known substances.
13. Which of the following noble gases is most soluble in water?
Explanation: Of all the noble gases, radon is the most soluble in water. Noble gases are non-polar substance, however, when dissolved in water, they are slightly soluble. This is due to dipole induced dipole interactions.
14. Which of the following noble gases can diffuse through rubber?
Explanation: Hydrogen is the lightest and the smallest noble gas, having an atomic number of 2. Owing to its small size and inert nature, it has the unusual property of diffusing through most commonly used laboratory materials such as rubber, glass or plastic.
15. Which among the following noble gases does not form clathrates?
Explanation: Noble gases can form compounds in which the gases are entrapped in the cavities of crystal lattices. Such compounds are called clathrates. Only Argon, Krypton, Xenon and Radon are known to form clathrates among the noble gases.
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