Chapter 5: States of Matter

Q24Written answer

Two different gases ‘A’ and ‘B’ are filled in separate containers of equal capacity under the same conditions of temperature and pressure. On increasing the pressure slightly the gas ‘A’ liquefies but gas B does not liquefy even on applying high pressure until it is cooled. Explain this phenomenon.

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Worked solution

The critical temperature is the term used for this phenomenon. Here gas A liquefies means that A is below its critical temperature and gas B does not liquefy on applying high pressure as it is above its critical temperature.

Q25Written answer

Value of universal gas constant (R) is the same for all gases. What is its physical significance?

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Worked solution

The dimensions of the universal gas constant R are energy per degree per mole. In the metre-kilogram-second system, the value of R is 8.3144598 joules per Kelvin per mole.

Hence R only depends on the unit of different parameter P, V, n, T. So R values the same for all gases.

Q26Written answer

One of the assumptions of the kinetic theory of gases states that “there is no force of attraction between the molecules of a gas.” How far is this statement correct? Is it possible to liquefy an ideal gas? Explain.

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Worked solution

The above statement is valid. At high-temperature gas, molecules movement become faster such that there is no intermolecular attraction. Under this condition, gas behave like an ideal gas. It is impossible to liquefy an ideal gas since ideal gas has no force of attraction between the gaseous molecules.

Q27Written answer

The magnitude of the surface tension of liquid depends on the attractive forces between the molecules. Arrange the following in increasing order of surface tension:

Water, alcohol (C2H5OH) and hexane [CH3(CH2)4CH3)].

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Worked solution

H-bonding is stronger in water than alcohol, so water has strong intermolecular attraction than alcohol. Increasing order of surface tension is – Hexane< alcohol< water.

Q28Written answer

The pressure exerted by saturated water vapour is called aqueous tension. What correction term will you apply to the total pressure to obtain a pressure of dry gas?

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Worked solution

The total pressure of the gas is Pmoist gas = Pdry gas

By applying the correction term,

Pdry gas = Pmoist gas – Aqueous tension

Therefore, the correction term applied to the total pressure of the gas to get the pressure of dry gas is P moist gas – aqueous tension

Q29Written answer

Name the energy which arises due to the motion of atoms or molecules in a body. How is this energy affected when the temperature is increased?

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Worked solution

Thermal energy arises due to the motion of atoms or molecules in the body. If we increase the temperature then the kinetic energy of atom and molecule increases significantly.

Q30Written answer

Name two intermolecular forces that exist between HF molecules in a liquid state.

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Worked solution

Hydrogen bonding and dipole-dipole interaction exist between HF molecule in a liquid state.

Q31Written answer

One of the assumptions of the kinetic theory of gases is that there is no force of attraction between the molecules of a gas. State and explain the evidence that shows that the assumption is not applicable for real gases.

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Worked solution

Under low pressure and at high temperature the assumption made by kinetic theory is true. At high temperature, the molecules will be very far from each other and at low pressure, the volume of molecule become negligible so they don’t interact and hence the assumption is valid at high temperature.

Q32Written answer

Compressibility factor, Z, of a gas is given as Z = PV/ nRT

(i) What is the value of Z for an ideal gas?

(ii) For real gas what will be the effect on the value of Z above Boyle’s temperature?

Solution;

(i) Compressibility factor, Z is defined as the ratio of the product of pressure and volume to the product of the number of moles, gas constant and temperature.

For an ideal gas, the value of Z is 1.

(ii) Above Boyle’s temperature, PV is greater than nRT for real gases so, the value of Z will also be greater than 1 i.e Z>1, which shows the positive deviation in the compressibility factor above Boyle’s temperature.

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Worked solution

Solution coming soon.

Q33Written answer

The critical temperature (T c) and critical pressure (Pc) of CO2 are 30.98°C and 73atm respectively. Can CO2(g) be liquefied at 32°C and 80atm pressure?

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Worked solution

CO2 gas cannot be liquefied at a temperature which is greater than its critical temperature i.e 30.98°C even by applying any pressure. So as the given temperature is 32°C by applying a pressure of 80atm the CO2 gas cannot be liquefied.

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II. Multiple Choice Questions (Type-II)