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Q1. If a graph is plotted between volume V v/s temperature ^oC at constant pressure. At what temperatures will it cut the volume and temperature axis?

Solution

The graph cuts the volume axis at 0^oC and the temperature axis at -273.15 ^oC, which is also known as the absolute temperature.

Q2. Explain why water has higher vapour pressure than mercury

Solution

Water has weaker inter-particle force. Hence, the Water has a higher vapour pressure than mercury.

Q3. Why do the falling liquid drops assume a spherical shape?

Solution

Liquid drop always assume a spherical shape. This is because drop always tends to acquire minimum surface area due to the surface tension, and for a given volume, the surface area of sphere is minimum.

Q4. What is the other name for Intermolecular Forces?

Solution

The other name for Intermolecular Forces is van der Waals forces.

Q5. Which has a higher boiling point I₂ or Br₂?

Solution

I₂ has a higher boiling point than Br₂. The atomic weights of Br and I are 80 and 127 respectively. Since I₂ has higher molecular weight, it has stronger London dispersion forces so it has a higher boiling point than Br₂.

Q6. Convert 25^oC into Kelvin scale.

Solution

K = ^oC + 273.15    = 25^oC + 273    = 298.15 K

Q7. Give reason for the high compressibility of gases on the basis of kinetic molecular theory?

Solution

According to the kinetic molecular theory, high compressibility of gases is due to large empty spaces between the molecules.

Q8. Name the type of forces present in CH₃Cl and PCl₃ molecule.

Solution

CH₃Cl and PCl₃ molecules have permanent net dipoles so they have dipole-dipole interactions.

Q9. Which should have a lower boiling point H₂O or H₂S?

Solution

H₂S have a lower boiling point i.e. -70^oC because no hydrogen bonding is present in liquid H₂S whereas the boiling point of water is 100^oC due to the presence of very strong hydrogen bonding.

Q10. 5.0g of an ideal gas occupies 9.2 L at STP. What volume would it occupy at 120°C and 92 mm Hg?

Solution

PV = nRT At STP, P = 760mm Hg, T = 273 K P₁ = 760 mm V₁ = 9.2L T₁= 273K P₁= 92 mm V₂ = ? T₂ = 120 + 273 = 393K Using relation:  

Q11. What is the freezing and boiling points of water at 1 atm pressure in Fahrenheit scale?

Solution

The freezing and boiling points of water at 1 atm are 32^oF and 212^oF respectively according to the Fahrenheit scale.

Q12. A closed gas system initially has pressure and temperature of 86.6torr and 424K with the volume unknown. If the same closed system has values of 597torr, 1240mL and 455K, what was the initial volume in mL?

Solution

P₁ = 86.6 torr V₁ = ? T₁= 424K P₁= 597 torr V₂ = 1240 mL T₂ = 455K Using relation:  

Q13. At what pressure would 0.450 mole of nitrogen gas at 23.0 °C occupy 8.90 L?

Solution

P =? V = 8.90 L n = 0.450 mol R = 0.0821 L atm mol^-1 K^-1 T = 23 + 273 = 296 K

Q14. Which substances among the following experiences dipole-dipole intermolecular forces? SiF₄, CHCl₃, CO₂, SO₂

Solution

SO₄ and CHCl₃ experience dipole-dipole intermolecular forces.

Q15. A sample of ideal gas occupies a volume of 238 mL at STP. To what temperature must the sample be heated if it is to occupy a volume of 185 mL at 2.25 atm?

Solution

At STP, P= 1 atm, T = 273 K P₁ = 1 V₁ = 238 mL T₁= 273K P₁= 2.25 atm V₂ = 185 mL T₂ = ? Using relation:  

Q16. Arrange the following forces in increasing order of their strength - Dipole-dipole interaction, hydrogen bond and dispersion forces.

Solution

Hydrogen bond > Dipole-dipole interaction > Dispersion force

Q17. What would happen if the molecular collisions in the kinetic theory are inelastic?

Solution

If the molecular collisions in the kinetic theory are inelastic, there would be a constant loss of energy. Due to this loss of energy, the molecular motion will slow down and ultimately they will settle down at the bottom.

Q18. If 9.006 grams of a gas are enclosed in a 50.00 liter vessel at 273.15 K and 2.000 atmospheres of pressure, what is the molar mass of the gas? What gas is this?

Solution

P = 2.000 atm V = 50.00L R = 0.0821 L atm mol^-1 K^-1 T = 273 K We know, The answer (2.019 g mol^-1) is approximately that of hydrogen gas, H₂.    

Q19. Differentiate between Dipole-dipole forces and Dipole-induced dipole forces.

Solution

Dipole-dipole forces Dipole-induced dipole forces i) Dipole-dipole forces act between molecules with permanent dipole. i) Dipole-induced dipole force is present between the polar molecule having permanent dipole and the molecule lacking permanent dipole. ii) Greater the dipole moment of polar molecule greater is the magnitude of dipole-dipole forces. This effect is known as Orientation Effect.  ii) As the size of the molecule or atom increases, the influence of permanent electrical dipole on it will also increase, resulting in the increase in magnitude of these forces. This effect is termed as Induction Effect.

Q20. How does the molecular shape of a molecule affect the strength of Dispersion forces?

Solution

The shapes of the molecules affect the strength of dispersion forces. Long thin molecules can develop bigger temporary dipoles due to electron movement than the short molecules containing same number of electrons. Long thin molecules also lie closer together. For example butane and 2-methyl propane has a molecular formula C₄H₁₀ but the atoms are arranged differently in both. In butane the carbon atoms are arranged in a single chain, but 2-methyl propane is a shorter chain with a branch. So butane has a higher boiling point because the dispersion forces are greater. The molecules are larger so they set up temporary dipole and can lie closer together than the shorter and fatter 2-methyl propane molecules.

Q21. Calculate the pressure of 1 x 10²¹ molecules of nitrogen dioxide when enclosed in a vessel of capacity of 2.5 L capacity at temperature 27ºC?

Solution

Now, PV = nRT P = ? atm V = 2.5 L R = 0.0821 L atm mol^-1 K^-1  T = 27 + 273 = 300 K n = 1.66 x 10^-3 mol P = 1.6 35 x 10^-2 atm  

Q22. Name the factors that affect the Viscosity of a substance?

Solution

The factors that affect the Viscosity of a substance are its molar mass, intermolecular forces, molecular shape and its temperature.

Q23. Out of fluorine and nitrogen monoxide which one has the higher melting and boiling points? Explain.

Solution

Nitrogen monoxide has higher melting and boiling points because nitrogen monoxide is a polar molecule with the strong Dipole-Dipole attraction forces.

Q24. Non-polar molecules do not have dipoles like polar molecules but still they form solids or liquids. Explain

Solution

• Atoms of non-polar molecules are electrically symmetrical. 
• But unsymmetrical charge distribution takes place due to the rapid motion of electrons; a momentary accumulation of electron density occurs on one side of the atom. This causes the development of temporary instantaneous dipole on the atom.
• This temporary dipole formed in one atom or molecules induces a dipole in the other atom. The temporary dipoles of both the atoms attract each other and this temporary force of attraction between two temporary dipoles is known as London force.
• As a result of this force, the non-polar molecules also form solids or liquids.

Q25. Which postulates of kinetic theory are not valid at high pressure and low temperature? Give reasons to justify their invalidity?

Solution

The postulates of the kinetic theory that are not valid at high pressure and low temperature are:

There is no force of attraction between the molecules of a gas.

Volume of the molecules of a gas is negligibly small in comparison to the space occupied by the gas.

According to the first assumption the gas will never liquefy but the gases do liquefy when cooled and compressed. According to the second assumption the plot of P Vs V at constant temperature based on the experimental data and that based on theoretical calculations from Boyle’s law should coincide. But at high pressures deviations are observed which confirms that these postulates of the kinetic theory are invalid.

Q26. Why does mercury have concave-downward meniscus?

Solution

In mercury, the cohesive forces are stronger than adhesive forces. Therefore, mercury have concave-downward meniscus.

Q27. What is the SI unit of temperature?

Solution

The S.I unit of temperature is Kelvin.

Q28. What is the relation between the deviations shown by the molecules of a gas and their molecular volume and attraction?

Solution

The gases show negative deviations (Z<1) when the effect of molecular attraction predominates. Similarly, positive deviations (Z >1) are shown when the effect of molecular volume dominates.

Q29. Give reasons for the following: (i) Glycerine is more viscous than water (ii) Evaporation causes cooling

Solution

(i) Glycerine has stronger intermolecular forces than water hence, it is more viscous than water. (ii) Evaporation causes cooling because the molecules that undergo evaporation are high energy molecules so the kinetic energy of molecules which are left behind is less. Since the average kinetic energy of the remaining molecules is less the temperature of these molecules is also lower. If temperature is kept constant the distribution of kinetic energy will be same. The high energy molecules keep on escaping from the liquid into the gas phase. This phenomenon brings a cooling effect.

Q30. What factors are responsible for the existence of matter in three states?

Solution

The balance between intermolecular forces and the thermal energy of the molecules are the factors responsible for the existence of matter in three states

Q31. A 30.0 g gas sample occupies 11.2 L at STP. Find the molecular weight of this gas.

Solution

One mole of a gas occupies a volume of 22.4 L at STP. This is a standard fact. 11.2 L at STP is one-half molar volume; therefore there is 0.5 mol of gas present. Thus, the molecular weight of the gas is 80.0 g mol^-1.  

Q32. What is Capillary action Explain?

Solution

Capillary action is the ability of a substance to flow up in a narrow tube against the force of gravity. It results from the combination of two forces the attraction force between molecules in a liquid called cohesive force and the attraction between the molecules in a liquid and the molecules of the surface of the tube called as adhesive force.

Q33. What types of intermolecular forces exist between the molecules of acetic acid (CH₃COOH) and carbon tetrachloride (CCl₄)?

Solution

In acetic acid (CH₃COOH), hydrogen bonding, dipole-dipole interactions and dispersion force are present whereas in carbon tetrachloride (CCl₄) only dispersion non-polar forces are present.

Q34. What is the relationship between pressure and density of a gas?

Solution

According to the Boyles law, at constant temperature the pressure is directly proportional to the density of a fixed mass of the gas.

Q35. Define coefficient of Viscosity.

Solution

Co-efficient of viscosity is the force of friction required to maintain velocity difference of 1cm sec^-1 between two parallel layers 1cm apart and having an area of 1cm². Or The ratio of the shearing stress to the velocity gradient is a measure of the viscosity of the fluid and is called the coefficient of viscosity. ‘η’ is measure of viscosity. SI unit of viscosity coefficient is 1 newton second per square metre (N s m^–2) = pascal second (Pa s = 1kg m^–1s^–1). In cgs system the unit of coefficient of viscosity is poise.

Q36. How gases can be liquefied?

Solution

Gases can be liquefied by compressing it and by decreasing the thermal energy of their molecules by lowering the temperature.

Q37. In the van der Waals equation, what do the constants 'a' and 'b' represent?

Solution

The constant 'a' gives the idea of the magnitude of attractive forces between the molecules of the gas and 'b' is the measure of effective volume occupied by the gas molecules in the van der Waals equation. It is also called co-volume or excluded volume.

Q38. What volume is needed to store 0.050 moles of helium gas at 202.6 kPa and 127ºC?

Solution

We know that, PV = nRT P = 202.6 kPa n = 0.050 mol T = 127 + 273 = 400K V = ? L R = 8.314 J K^-1 mol^-1 202.6 x V = 0.050 x 8.314 x 400 202.6 V = 166.28 V = 0.821 L or 821 mL

Q39. Name the type of forces present in Cl₂ and H₂ molecules.

Solution

Cl₂ and H₂ are non-polar molecules. Hence, London forces are present in them.

Q40. Define absolute zero.

Solution

The lowest imaginary temperature at which gases are supposed to occupy zero volume is called absolute zero.