# First Year Chemistry Gases 8. A 12.5 \mathrm{dm}^{3} vessel contains 4.0 \mathrm{~g} \mathrm{CH}_{4} 1.8 \mathrm{~g} \mathrm{~N}_{2} and 10.0 \mathrm{gX} \mathrm{e} . What is the pressure in the vessel at 0^{\circ} \mathr

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##### 8. A 12.5 \mathrm{dm}^{3} vessel contains 4.0 \mathrm{~g} \mathrm{CH}_{4} 1.8 \mathrm{~g} \mathrm{~N}_{2} and 10.0 \mathrm{gX} \mathrm{e} . What is the pressure in the vessel at 0^{\circ} \mathrm{C} ? (0.698 \mathrm{~atm}) Ans.(Hint: Find out the total moles of the gases and then apply \mathrm{PV}=\mathrm{nRT} ).

(b) Compare the rates of diffusion of the following pairs of gases:(i) H_{2} and D_{0} :

Q15 Explain the following facts(iv) Water vapours do not behave ideally at 273 \mathrm{~K} .

Q8. Derive the units for gas constant R in general gas cquation: when energy is expressed in ergs.

Fill in the blanks.(i). The product PV has the S.I. unit of(ii). Eight grams each of \mathrm{O}_{2} and \mathrm{H}_{2} at 27^{\circ} \mathrm{C} will have total \mathrm{K} . \mathrm{E} in the ratio of(iii). Smell of the cooking gas during leakage from a gas cylinder is due to the property of of gases.(iv). Equal of ideal gases at the same temperature and pressure contain number of molecules.(v). The temperature above which a substance exists only as a gas is called

7. 1.40 \mathrm{dm}^{3} volume of a gas measured at a temperature of 27^{\circ} \mathrm{C} and a pressure of 900 torr was found to have mass 2.273 \mathrm{~g} . Calculate the molecular mass of the gas [Hint: Calculate " n " by the formula PV=nRT and then molecular mass] (33.72 amu) Ans.

Q21.a. The relative densities of two gases \mathrm{A} and \mathrm{B} are 1: 1.5 . Find out the volume of B which will diffuse in the same time in which 150 \mathrm{dm}^{3} of A will diffuse?

b. How does kinetic molecular theory of gases explain the following gas laws:(ii) Charless law

Select the correct answer out of the following alternative suggestions.Pressure remaining constant at which temperature the volume of a gas will become twice of what it is at 0^{\circ} \mathrm{C} .a. 546^{\circ} \mathrm{C} b. 200^{\circ} \mathrm{C} c. 546 \mathrm{~K} d. 273 \mathrm{~K}

Select the correct answer out of the following alternative suggestions.) Equal masses of methane and oxygen are mixed in an empty container at 25^{\circ} \mathrm{C} . The fraction of total pressure exerted by oxygen is:a. \frac{1}{3} b. \frac{8}{9} \frac{1}{9} d. \frac{16}{17}

21. Explain the following:(i) A falling drop of a liquid is spherical.

b. Can we determine the molecular mass of an unknown gas if we know the pressure temperature and volume along with the mass of that gas.

Q12. a.What is kinetic molecular theory of gases? Give its postulates.

Q23 Calculate the masses of 10^{20} molecules of each of \mathrm{H}_{2} \mathrm{O}_{2} and \mathrm{CO}_{2} at STP. What will happen to the masses of these gases when the temperature of these gases are increased by 100^{\circ} \mathrm{C} and the pressure is decreased by 100 torr?

d. Why do we feel comfortable in expressing the densities of gases in the units of \mathrm{g} \mathrm{dm}^{-3} rather than \mathrm{g} \mathrm{cm}^{-3} a unit which is used to express the densities of liquids and solids.

c. Do you expect the same decrease in the pressure of two moles of \mathrm{NH}_{3} having a volume of 40 \mathrm{dm}^{3} and at temperature of 27^{\circ} \mathrm{C} .

c. The rate of effusion of an unknown gas A through a pinhole is found to be 0.279 times the rate of effusion of \mathrm{H}_{2} gas through the same pinhole. Calculate the molecular mass of the unknown gas at STP.(Ans: =25.7 \mathrm{gmol}^{-1} )

c. How do you justify from general gas equation that increase in temperature or decrease of pressure decreases the density of the gas?

What will be the volume occupied by 14 \mathrm{~g} of nitrogen at 20^{\circ} \mathrm{C} and 740 tor pressure?

14. Describe gases liquids and solids on a molecular basis and explain their behaviour and change of state.

Q16 Helium gas in a 100 \mathrm{~cm}^{3} container at a pressure. of 500 torr is transferred to a container with a volume of 250 \mathrm{~cm}^{3} . What will be the new pressure:b. if its temperature changes from 20^{\circ} \mathrm{C} to 15^{\circ} \mathrm{C} ?

Q22 Calculate the number of molecules and the number of atoms in the given amounts of each gas(b) 1 \mathrm{~cm}^{3} of \mathrm{NH}_{3} at 100^{\circ} \mathrm{C} and pressure of 1.5 atm \left(\right. Ans: 2.94 \times 10^{10} 1.177 \times 10^{20} )

15. Co-relate the energy changes accompanying changes of state.

Q13. a. Gases show non-ideal behaviour at low temperature and high pressure. Explain this with the help of a graph.

Q11. a. What is critical temperature of a gas? What is its importance for liquefaction of gases? Discuss Lindes method of liquefaction of gases.

b. How does kinetic molecular theory of gases explain the following gas laws:(iv) Grahams law of diffusion

21. Explain the following:(vi) Mercury has its meniscus upward.